Digitalphilologie

Titel

On Rock Formations

Jahr

1824

Ort

Edinburgh

Nachweis

in: The Edinburgh Philosophical Journal 10:19 (Januar 1824), S. 40–53, 10:20 (April 1824), S. 224–239.

Sprache

Englisch

Typografischer Befund

Antiqua; Auszeichnung: Kursivierung, Kapitälchen; Fußnoten mit Asterisken; Schmuck: Initialen; Besonderes: mathematische Sonderzeichen.

Identifikation

Textnummer Druckausgabe: IV.26
Dateiname: 1822-Independance_des_formations-2

Statistiken

Seitenanzahl:	30
Zeichenanzahl:	67073

Weitere Fassungen
	Indépendance des formations (Paris; Strasbourg, 1822, Französisch)
	On Rock Formations (Edinburgh, 1824, Englisch)
	On Rock Formations (Boston, Massachusetts, 1824, Englisch)
	О волканическихъ областяхъ [O volkaničestkich oblastjach] (Sankt Petersburg, 1832, Russisch)
	Description of the Muschelkalk and Quadersandstein (London, 1836, Englisch)

|40|

On Rock Formations. By Baron AlexanderHumboldt *.

THE word formation designates, in Geognosy, either the man-ner in which a rock has been produced, or an assemblage ofmineral masses, which are so connected together, that they aresupposed to have been formed at the same period, and present,in the most distant parts of the world, the same general rela-tions of situation and position. It is thus that the formationof obsidian and basalt is attributed to subterranean fires; andthus also that we say the formation of transition clay-slate con-tains lydian-stone, chiastolite, alum-slate, and alternating beds ofblack limestone and porphyry. The first acceptation of theword is better adapted to the genius of the language; but ithas relation to the origin of things, to an uncertain science found-ed upon geogonic hypothesis. The second acceptation, now

* Translated from Essai Geognostique par Alexandre de Humboldt.|41| generally adopted by the French mineralogists, has been bor-rowed from the celebrated school of Werner: it indicates whatis, not what is supposed to have been.In the geognostical description of the globe, we may distin-guish different degrees of aggregation of mineral substances,simple or compound, according as we rise to more general ideas.Rocks which alternate with one another, which are usually asso-ciated, and which present the same relations of position, consti-tute a formation; the union of several formations constitutes adistrict or terrain; but these different terms of rocks, Forma-tion and Terrain, are employed as synonymous in many worksof geognosy.The diversity of the rocks, and the relative disposition of thebeds which form the oxidised crust of the earth, have, from themost remote times, fixed the attention of men. Wherever theworking of a mine was directed upon a deposit of salt, of coal,or of clay-iron, which was covered with a great number of bedsof different natures, it gave rise to ideas more or less precise re-garding the system of rocks peculiar to a district of small ex-tent. Furnished with these local details, and full of prejudiceswhich arise from custom, the miners of a country would dispersethemselves over the neighbouring districts. They would dowhat geognosts have often done in our days; they would judgeof the position of rocks of whose nature they were ignorant, ac-cording to imperfect analogies, according to the circumscribedideas which they had acquired in their native country. Thiserror must have had a fatal influence upon the success oftheir new researches. In place of examining the connectionof two contiguous districts, by following some generally ex-tended bed,—in place of enlarging and extending, so to speak,the first type of formations which had remained impressedupon their minds,—they would be persuaded that each por-tion of the globe had an entirely different geological constitu-tion. This very old popular opinion has been adopted and sup-ported, in different countries, by very distinguished men; butsince geognosy has been elevated to the rank of a science, theart of interrogating nature brought to perfection, and journeysmade into distant countries, have presented a more exact com-parison of different districts, great and immutable laws havebeen discovered in the structure of the globe, and in the super-|42| position of rocks. Since, then, the most striking analogies of situa-tion, of composition, and of organic bodies contained in contem-poraneous beds, have manifested themselves in the two worlds, inproportion as we become accustomed to consider the formationsunder a more general point of view, even their identity becomesevery day more probable.In fact, on examining the solid mass of our planet, we per-ceive that some of those substances with which oryctognosy (de-scriptive mineralogy), makes us acquainted in their individualcapacities, are met with in constant associations, and that theseassociations, which are designated by the name of CompoundRocks, do not vary, like organic beings, according to the diffe-rences of the latitudes, or of the isothermal lines in which theyoccur. The geognosts who have travelled over the most re-mote countries, have not only met in the two hemispheres withthe same simple substances, quartz, felspar, mica, garnet or horn-blende; but they have also found that the great mountain mas-ses present almost everywhere the same rocks, that is to say, thesame assemblages of mica, quartz and felspar in the granite; ofmica, quartz, and garnets in the mica-slate; of felspar and horn-blende in the syenite. If it has sometimes been thought at firstthat a rock belonged exclusively to a single portion of the globe,it has been constantly found by later researches, in regions themost remote from its first locality. We are tempted to admitthat the formation of rocks has been independent of the diver-sity of climates; that perhaps it has even been anterior to them,(Humboldt, Geographie des Plantes, 1807, p. 115.; Vues desCordilleres, vol. i. p. 122). Rocks are found to be identicalwhere organic beings have undergone the most varied modifica-tions.But this identity of composition, this analogy which is obser-ved in the association of certain simple mineral substances, mightbe independent of the analogy of relative situation and of super-position. One may have brought from the Islands of thePacific Ocean, or from the Cordilleras of the Andes, the samerocks which are observed in Europe, without his being permit-ted to conclude that these rocks are superimposed in the samemanner, and that after the discovery of one of them it might bepredicted with some degree of certainty what are the other rocks|43| which occur in the same places. It is to discover these analo-gics of situation and relative position, that the labours of geo-gnosts should tend, who delight to investigate the laws of inor-ganic nature. In the following tables, we have attempted tounite all that is known with certainty, regarding the superposi-tion of rocks in the two Continents, to the north and south ofthe Equator. These types of formations will not only be extend-ed, but also variously modified, in proportion as the number oftravellers qualified to make geognostical observations shall be-come increased, and as complete monographs of different dis-tricts at great distances from each other shall furnish more pre-cise results.The exposition of the laws observed in the superposition ofrocks, forms the most solid part of the science of geognosy. Itmust not be denied, that the observations of geognostical situa-tion often present great difficulties, when the point of contact oftwo neighbouring formations cannot be reached, or when theydo not present a regular stratification, or when their relative situa-tion is not uniform, that is to say, when the strata of the upperdeposits are not parallel to the strata of the lower. But thesedifficulties (and this is one of the great advantages of observa-tions which embrace a considerable part of our planet), diminishin number, or disappear entirely, on comparing several districtsof great extent. The superposition and relative age of rocks,are facts susceptible of being established immediately, like thestructure of the organs of a vegetable, like the proportions ofelements in chemical analysis, or like the elevation of a moun-tain above the level of the sea. True geognosy makes knownthe outer crust of the globe, such as it exists at the presentday. It is a science as capable of certainty as any of the physi-cal descriptive sciences can be. On the other hand, all that re-lates to the ancient state of our planets, to those fluids which, itis said, held all the mineral substances in a state of revolution,to those seas which we have raised to the summit of the Cordil-leras, to make them again disappear, is as uncertain as are theformation of the atmosphere of planets, the migrations of vege-tables, and the origin of different varieties of our species. Yetthe period is not very remote when geologists occupied them-selves by preference with the solution of these almost impossi-|44| ble problems, with those fabulous times of the physical historyof our planet.In order to render the principles better understood, accord-ing to which the following table of the superposition of rocks isconstructed, it becomes necessary to premise observations fur-nished by the practical examination of different districts. Weshall begin with remarking, that it is not easy to circum-scribe the limits of a formation. The Jura limestone and theAlpine limestone, which are separated to a great distance in onecountry, sometimes appear closely connected in another. Whatannounces the independence of a formation, as has been veryjustly observed by M. de Buch, is its immediate superpositionupon rocks of a different nature, and which consequently oughtto be considered as more ancient. The red sandstone is an in-dependent formation, because it is superimposed indifferentlyupon black (transition) limestone, upon mica-slate, or uponprimitive granites; but in a country where the great formationof syenite and porphyry predominates, these two rocks constant-ly alternate. There results that the syenite rock is dependentupon the porphyry, and scarcely any where covers by itself thetransition clay-slate or primitive gneiss. The independence offormations does not, besides, by any means exclude the uniformi-ty or concordance of position; it rather excludes the oryctognos-tic passage of two superimposed formations. The transition dis-tricts have very often the same direction and the same inclina-tion as the primitive ones; and yet, whatever approximationthere may be between their origin, we are not the less warrant-ed to consider the anthracitic mica-slate or the grey-wacke, al-ternating with porphyry, as two formations independent of theprimitive granites and gneisses which they cover. The confor-mity of position is in no way incompatible with the indepen-dence of formations, that is to say, it does not prevent the rightwhich one has of regarding a rock as a distinct formation. Itis because the independent formations are placed indifferentlyon all the older rocks, (the chalk upon the granite, the red sand-stone upon the primitive mica-slate), that the assemblage of agreat number of observations made upon very distant points,becomes eminently useful in the determination of the relativeage of rocks. In order to determine that the zircon-sienite is|45| a transition rock, it must have been seen resting upon formationsposterior to the black limestone with orthoceratites. Observationsmade upon the porphyries and syenites of Hungary by M. Beu-dant, one of the most distinguished geologists of the presenttimes, may throw much light upon the formations of the Mexi-can Andes. It is thus that a new vegetable discovered in In-dia, elucidated the natural affinity between two families of plantsbelonging to Equinoctial America.The order which has been followed in the table of formations,is that of the situation and relative position of rocks. I do notpretend that this position is observed in all the countries of theglobe; I merely point it out such as it has appeared the mostprobable, after the comparison of a great number of facts whichI have collected. It is by the idea of the relative age of for-mation, that I have been guided in this work, imperfect as itstill is. I had begun it long before my journey to the Cordille-ras of the New Continent, from the year 1792, when, on leav-ing the Freyberg School, I was appointed to the direction ofthe Mines in the mountains of the Fichtelgebirge. The samerock may vary in composition, integrant parts may have beenabstracted, and new substances may occur disseminated, withoutthe rock’s changing its denomination in the eyes of the geognostwho is engaged with the superposition of formations. Un-der the equator, as in the north of Europe, strata of a true tran-sition syenite lose their hornblende, without the mass becom-ing another rock. The granites of the banks of the Ori-noco sometimes assume hornblende as an integrant part, andyet do not cease to be primitive granite, although this may notbe of the first or oldest formation. These facts have been ob-served by all practical geologists. The essential character ofthe identity of an independent formation is its relative position,the place which it occupies in the general series of formations.(See the classical Memoir of M. de Buch, Ueber den Begriffeiner, in the Mag. der Naturf., 1810, p. 128–133.) It is onthis account that an isolated fragment, a specimen of rock foundin a collection, cannot be determined geognostically, that is tosay, it cannot be referred with certainty to a particular forma-tion, constituting one of the numerous beds of which the crustof our planet is composed. The presence of chiastolite, the ac-|46| cumulation of carbon or nodules of compact limestone in theclay-slates, nigrine and epidote in the syenites, (alternating withgranite and porphyries), conglomerates or pudding-stones, ha-ving a basis of anthracitic mica-slate, may, without doubt, becharacteristic of transition formations; in the same way as, ac-cording to the useful labours of M. Brongniart, petrifactions ofshells, in a good state of preservation, sometimes indicate direct-ly such or such a bed of tertiary deposits. But these cases,where we are guided by disseminated substances or by charac-ters purely geological, comprehend but a small number ofrocks of a recent origin, and observations of this kind often leadonly to negative facts. The characters taken from the colour,from the grain, and from small veins of carbonate of lime,which traverse calcareous rocks; those which are furnished bythe fissility and silky lustre of clay-slate, the aspect and undu-lations more or less marked of the scales of mica in mica-slate;and, lastly, the size and colouring of the crystals of felspar inthe granites of different formations, may, like all that is con-nected simply with the physiognomy of minerals, lead the mostexpert observer into error. The white and black tints un-doubtedly in most instances distinguish the primitive and tran-sition limestones; the Jura formation, especially in its upperbeds, is also without doubt generally divided into thin whitishbeds, having a dull, even, or conchoidal fracture, with veryflat cavities: but in the mountains of transition limestone thereare isolated masses which, in colour and texture, resemble theoryctognostic characters of the Jura limestone; and to the southof the Alps there are hills belonging to tertiary deposits, wherewe find rocks analogous to the slaty and dull Jura limestone(in as far as regards appearance,) in formations placed above thechalk, and which resemble the limestone used for lithographicpurposes. Were names taken from their oryctognostical cha-racters alone to be preferred in distinguishing formations, thedifferent strata of the same compound rock having a consider-able thickness, and extended to a great length in a particulardirection, would often seem to belong to different rocks, accord-ing to the points at which specimens were taken. Consequent-ly we can only determine geognostically in collections, suites ofrocks of which the mutual superposition is known.|47| In announcing these ideas regarding the sense which shouldbe attached to the words independent formations, when treatingof the order of their position, we are very far from undervaluingthe eminent services which the most rigorous oryctognostic exa-mination, the minute investigation of the composition of rocks,have rendered to modern geognosy, and especially to the know-ledge of the relative position of formations. Although, accord-ing to the discoveries of M. Haüy, regarding the intimatenature of inorganic and crystallized substances, there does notexist, properly speaking, a passage or transition of one mineralspecies to another; (Cordier, sur les Roches volcan., p. 33., andBerzelius, Nouv. Syst. de Mineral, p. 119.), the passages ofmasses or pastes of rocks, are not limited to formations whichare commonly distinguished by the name of Compound Rocks.Those which are thought simple, for example, the transition orsecondary limestones, are partly amorphous varieties of mineralspecies, of which there exists a crystallized type, partly of aggre-gates of clay, carbon, &c., which cannot be submitted to anyfixed determination. It is upon the variable proportions ofthese heterogeneous mixtures, that the passage of marly lime-stones to other schistose formations is founded. (Haüy, Tableaucomparatif de la Cristallographie, p. 27.—30.) All the amor-phous pastes of rocks, however homogeneous they appear at firstsight, the bases of porphyries and euphotides (serpentines), aswell as those problematical black masses which constitute the ba-sanite (basalt) of the ancients, and which are not all greenstonessurcharged with hornblende, are susceptible of being subjected tomechanical analysis. M. Cordier has applied this analysis in aningenious manner to the diabases, dolerites, and other more re-cent volcanic productions. The most apparently minute orycto-gnostic examination, cannot be indifferent to the geognost who ex-amines the age of formations. It is by this examination that weare enabled to form a just idea of the progressive manner in which,by internal development, that is to say, by a very gradual changein the proportions of the elements of the mass, the passage ismade from one rock to a neighbouring. The transition slates,whose structure appears at first so different from that of the gra-nites or porphyries, present to the attentive observer striking ex-amples of insensible passages to granular rocks of porphyritic|48| or granitic nature. These slates become at first greenish andharder. In proportion as the amorphous paste receives horn-blende, it passes into those hornblendic traps which in formertimes were confounded with basalt. In other cases, the mica,which is at first concealed in the amorphous paste, becomes de-veloped, and separates into distinct and clearly crystallizedspangles; at the same time, the felspar and quartz become visi-ble; and the mass assumes a granular aspect, with very elon-gated grains: this is a true transition gneiss. By degrees, thegrains lose their common direction; the crystals arrange them-selves around many centres; the rock becomes a transition gra-nite or syenite. In other cases, the quartz alone is developed;it augments, and becomes rounded into nodules, and the slatepasses to the best characterized grey-wackes. By these certainsigns, geognosts, to whom the appearances of nature have becomefamiliar by long examination, become aware beforehand of theproximity of granular, granitic and arenaceous rocks. Analo-gous passages of primitive mica-slate to a porphyritic rock, andthe return of this rock to gneiss, are observed in the eastern partsof Switzerland. (See the luminous developments given by M.de Raumer, Fragmente, p. 10. and 47.; M. Leopold de Buch,in his Voyage de Glaris à Chiavenna, fait en 1803, and insertedin the Magaz. der Berl. Naturf., vol. iii. p. 115). But these pass-ages are not always insensible and progressive; the rocks oftenalso succeed each other quickly, and in a very abrupt manner;often (for example, at Mexico, between Guanaxuato and Ovex-eras), the limits between the slates, the porphyries and syenites,are as distinct as the limits between the porphyries and lime-stones; but even in this case, geognostical relations with the su-perimposed rocks are indicated by additional heterogeneousbeds. It is thus that the transition granite of the syenitic for-mation presents beds of basanite, by becoming charged withhornblende: it is thus, also, that these same granites sometimespass to euphotide. (Buch, Voyages en Norwege, vol. i. p. 138.,vol. ii. p. 83.)There results from these considerations, that the mechanicalanalysis of amorphous pastes, by means of demi-triturations andwashings, (an analysis of which, M. Fleuriau de Bellevue madethe first attempt, that was crowned with success; Journ. de|49| Physique, vol. li. p. 162.), throws light at once, 1st, Upon thelarge crystals which are isolated and separated from the micro-scopic crystals entangled in the mass; 2dly, On the mutual pass-ages of some rocks, superimposed the one upon the other; 3dly,On the subordinate beds, which are of the same nature as one ofthe elements of the amorphous mass. All these phenomena areproduced, if we may so speak, by internal development; byvariation in the constituent parts of a heterogeneous mass.Crystalline molecules, invisible to the eye, occur enlarged anddisengaged from the compact tissue of the paste; by their as-semblage and mixture with new substances, they insensibly be-come intercalated beds of considerable thickness; and not un-frequently they even become new rocks.It is the intercalated beds which especially merit the greatestattention. (Leonhard, Kopp and Gærtner, Propæd. der Miner.,p. 158.) When two formations succeed each other imme-diately, it happens that the beds of the one begin at first to alter-nate with the beds of the other, until (after these precursors ofa great change) the newest formation shows itself without anymixture of subordinate beds. (Buch, Geogn. Beob. vol. i.p. 104. 156.; Humboldt, Rel. Hist. vol. ii. p. 140.) The pro-gressive developments of the elements of a rock, may, conse-quently, have a great degree of influence upon the relative posi-tion of the mineral masses. Their effects belong to the provinceof geology; but, in order to discover and appreciate them, theobserver must call to his assistance oryctognosy.In exposing the intimate relations by which we often see thephenomena of composition connected with those of relative situa-tion, it has not been my intention to speak of the purely oryc-tognostic method, which considers rocks according to the ana-logy of their composition alone. (Journal des Mines, vol. xxxiv.No. 199.) In the classifications of this method, abstraction ismade of every idea of superposition; but they do not the lessgive rise to interesting observations regarding the constant assem-blage of certain minerals. A purely oryctognostic classification,multiplies the names of rocks more than is required by geognosy,when occupied with superposition alone. According to thechanges which the mixed rocks undergo, a stratum of great ex-tent and thickness may contain (we must repeat it here) parts|50| to which the oryctognost, who classes rocks according to theircomposition, would give entirely different denominations. Theseremarks have not escaped the learned author of the Classifica-tion Mineralogique des Roches; they must have presented them-selves to an experienced geognost, who has so successfully in-vestigated the superposition of the deposites of which he hastreated. “We must not confound,” says M. Brongniart, in hislate Memoir on the position of the Ophiolites, “the relative po-sitions, the orders of superposition of the deposites and of therocks which compose them, with purely mineralogical descrip-tions. The neglect of making the proper distinction in thiscase, would necessarily be productive of confusion in the science,and would retard its progress.” The arrangement which wegive at the end of this article, is by no means what is called aclassification of rocks; there will not even be found united, un-der the title of particular sections (as in the old geognosticalmethod of Werner, or in the excellent Traité de Geognosie ofM. D’Aubuisson), all the primitive formations of granite, norall the secondary formations of sandstone and limestone. Ithas been attempted, on the contrary, to place each rock as it oc-curs in nature, according to the order of its superposition or ofits respective age. The different formations of granite are se-parted by gneisses, mica-slates, black-limestones and grey-wackes. In the transition rocks, we have separated the forma-tions of porphyries and syenites of Mexico and Peru, which areanterior to the grey-wacke, and to the limestone with orthocera-tites, from the much more recent formation of the zircon-porphy-ries and syenites of Scandinavia. In the secondary rocks, we haveseparated the oolitic sandstone of Nebra, which is posterior tothe alpine limestone or zechstein, from the red-sandstone, whichbelongs to the same formation with the secondary porphyry andamygdaloid. According to the principle which we follow, thesame names of rocks occur several times in the same table. An-thracitic mica-slate is separated, by a great number of olderformations, from the mica-slate anterior to the primitive clay-slate.Instead of a classification of granitic, schistose, calcareous andarenaceous rocks, it has been my object to present a sketch ofthe geognostical structure of the globe; a table in which the|51| superimposed rocks succeed each other, from below upwards, asin those ideal sections which I designed in 1804, for the benefitof the Mexican School of Mines, and of which many copieshave been distributed since my return to Europe. (Bosquejode una Pasigrafia geognostica, con tablas que ensenan la es-tratificacion y el parallelismo de las rocas en ambos Continentes,para el uso del Real Seminario de Mineria de Mexico.) ThesePasigraphic tables united to my own observations made in bothAmericas what had at that period been known with precision re-garding the relative position of the primitive, intermediary andsecondary rocks in the Old Continent. They presented, togetherwith the type which might be considered as the most general,the secondary types, that is to say, the beds which I have namedparallel. This same method has been followed in the workwhich I now publish. My parallel formations are geognosticalequivalents; they are rocks which represent each other. (See theTraité de Geologie de M. d’Aubuisson, vol. ii. p. 255.) In Eng-land, and on the opposite Continent, there does not exist an iden-tity of all the formations: there exist equivalents or parallel for-mations. That of our coal situated between the transition massesand the red-sandstone, the position of the rock-salt which occurson the Continent in the alpine limestone, and the position of ouroolites in the Nebra sandstone and Jura limestone, may guidethe geognost in the approximation of remote formations. InEngland, we observe the coals placed upon transition formations;for example, upon the mountain-limestone of Derbyshire and ofSouth Wales, and upon the transition sandstone, or old red-sandstone of Herefordshire. I have thought that I recognisedin the magnesian-limestone the red marl, the lias and whiteoolites of Bath, the united formations of the alpine limestone,of the oolitic sandstone and Jura limestone. In comparing theformations of countries more or less distant from each other,those of England and of France, for instance, of Mexico andHungary, of the secondary basin of Santa Fe de Bogota and ofThuringia, we must not think of opposing to each individualrock a parallel one; it must be recollected, that a single forma-tion may represent several others. It is according to this princi-ple that beds of clay, lying beneath the chalk, may, in France,|52| be separated in the most distinct manner from the oolitic lime-stone beds; while in Switzerland, in Germany, and in SouthAmerica, they have for equivalents beds of marls subordinate tothe Jura limestone. The gypsums, which, in one district, aresometimes only intercalated beds in the alpine limestone or ooli-tic sandstone, in another district, assume all the appearance ofindependent formations, and occur interposed between the alpinelimestone and the oolitic sandstone, between this sandstone andthe muschelkalk. The learned Oxford Professor, Mr Buckland,whose extensive researches have been equally useful to the geo-gnosts of England and of the Continent, has lately published atable of parallel formations, or, as he calls them, equivalents ofrocks, which only extends from the 44th to the 54th degree ofnorth latitude, but which merits the greatest attention. (On theStructure of the Alps, and their relation with the rocks of Eng-land, 1821.)As in the history of ancient nations, it is easier to verify theseries of events in each country, than to determine their mutualcoincidence; so also more accuracy can be attained in estima-ting the superposition of formations in isolated regions, than indetermining the relative age or parallelism of formations whichbelong to different systems of rocks. Even in countries whichare not widely separated, in France, in Switzerland, and in Ger-many, it is not easy to fix the relative antiquity of the muschel-kalk, of the molasse of Argovie, and of the quadersandstein ofthe Hartz; because rocks of general occurrence are here mostcommonly wanting, which, according to the happy expression ofof M. de Grüner, serve as a geognostical horizon, and withwhich we might compare the three formations in question.When rocks are not in immediate contact, we can only judge oftheir parallelism by the relations of age existing between themand other formations by which they are united.These inquiries of comparative geognosy, will long occu-py the sagacity of observers; and it is not surprising that thosewho set out with the idea of retracing each formation in all theindividuality of its relative position, interior structure and subor-dinate beds, should finish with utterly denying all analogy ofsuperposition. I had the advantage of visiting, previous to myjourney to the Equator, a great part of Germany, of France, of|53| Switzerland, of England, of Italy, of Poland, and of Spain. Du-ring these excursions, my attention was particularly directed tothe relative position of formations, a phenomenon which I cal-culated upon discussing in a special work. On my arrival inSouth America, and while at first traversing in different direc-tions the vast deposites which stretch from the maritime chain ofVenezuela to the basin of the Amazon, I was singularly struckwith the conformity of position which the two Continents present.(See my first sketch of a Geological Table of Equinoctial Ameri-ca, in the Journal de Phys., vol. liii. p. 38.) Subsequent obser-vations, which included the Cordilleras of Mexico, of New Grena-da, of Quito, and of Peru, from the 21st degree of north latitudeto the 12th degree of south latitude, have confirmed these firstperceptions. But in speaking of analogies which are observedin the relative position of rocks, and of the uniformity of thoselaws which reveal to us the order of Nature, I might adduce a tes-timony otherwise of more weight than mine, that of the greatgeognost whose works have thrown the greatest light upon thestructure of our globe. M. Leopold de Buch has pushed hisresearches from the Archipelago of the Canary Isles to beyondthe Polar Circle to the 71st degree of latitude. He has discoverednew formations situated between others already known; and, inthe primitive as in the transition deposites, in the secondary as wellas in the volcanic, he has been struck with the great features bywhich the table of formations is characterized in the most distantregions.(To be continued.)|224|

On Rock Formations. By Baron AlexanderHumboldt. (Concluded from p. 53.)

From that scepticism which would deny the existence of anykind of regular order in the position of rocks, it is proper to dis-tinguish an opinion which has sometimes found supportersamong experienced observers. According to this opinion, the for-mations of gneiss-granite, of greywacke, of alpine limestone,and of chalk, which have a uniform superposition in differentcountries, do not very well correspond among themselves as tothe age of the homonymous elements of each series. It is thoughtthat a secondary rock may have been formed on one spot of theglobe, while transition rocks did not yet exist on another spot.In this supposition, no allusion is had to those granitic rockswhich are found lying above limestone containing orthoceratites,and which are consequently newer than the primitive rocks. Itis a fact generally admitted at this day, that formations of ana-logous composition have been repeatedly deposited at epochs farremoved from each other. The doubt which we are now consi-dering, (though we do not partake in it), bears on a point muchless clearly established,—the ascertaining whether certain mica-slate rocks, evidently situate in the midst of a country of primi-|225| tive rocks, and placed below those in which the vestiges of or-ganic life begin to appear, are newer than the secondary rocksof another country. I confess, that, in the part of the globe whichI have had an opportunity of examining, I have not seen anything that might tend to confirm this opinion. Granular syeni-tic rocks repeated twice, perhaps even three times, in primitive,intermediary (and secondary) deposits, are analogous phenome-na with which we have become acquainted within these fifteenyears. The disagreement in regard to age of great homonymousformations, does not by any means seem to me to be proved asyet by direct observations, made upon the contact of superim-posed formations. The chalk or Jura limestone may, on onehand, immediately cover primitive granite, and, on the other, beseparated from it by numerous secondary and transition rocks:these very common facts demonstrate only the abstraction, theabsence, or non-development of several intermediary membersof the geognostical series. The greywacke may, on one hand,dip beneath a felspar rock, or rock of which felspar forms aprincipal constituent; for example, beneath transition granite orzircon syenite; and, on the other hand, be superimposed uponthe black limestone containing madrepores; but this positionshows only the intercalation of a bed of greywacke between fel-spar rocks. Since the minute investigation of fossil organic bo-dies has, through the important labours of Messrs Cuvier andBrongniart, diffused a new life as it were in the study of thetertiary formations, the discovery of the same fossils in analogousbeds of very distant countries, has rendered the isochronism ofvery generally extended formations still more probable.It is this isochronism alone, this admirable order of succession,which seems given to man to investigate with some degree ofcertainty. The attempts which theological geologists have madeto subject the periods to absolute measurements of time, and toconnect the chronology of ancient cosmogonic narrations withthe observations of nature, could not possibly have been produc-tive of satisfactory results. “It has been more than once,” saysM. Ramond, in a discourse replete with philosophical views,“thought that a supplement to our short annals might be foundin the monuments of Nature. There is, however, enough of his-torical ages, to let us see that the succession of physical and mo-|226| ral events, is not regulated by the uniform progress of time, andcould not consequently give it measure. We see behind us aseries of creations and destructions, by means of the strata ofwhich the crust of the earth is composed. They give rise to theidea of so many distinct periods; but these periods, so fertile inevents, may have been very short, in comparison with the num-ber and importance of the results. Between the creations anddestructions, on the other hand, we see nothing, however vastthe intervals may be. There, where all is lost in the void of un-determined antiquity, the degrees of relative age have no appre-ciable value; because the succession of phenomena has no longerthe scale which relates to the division of time.” (Memoires del’Institut, for the year 1815, p. 47.)In the geognostical monography of a deposit of small extent,for example, the environs of a city, one cannot distinguish withsufficient minuteness the different beds which compose the localformations, shelving banks of sand and clay, the subdivisions ofgypsums, the strata of marly and oölitic limestone, designatedin England by the names of Purbeck Beds, Portland Stone,Coral Rag, Kelloway Rock, and Corn Brash, then acquire a greatdegree of importance. Thin beds of secondary and tertiary for-mations, containing assemblages of very characteristic fossil bo-dies, have furnished, as it were, a horizon to the geognost. Intheir prolongation, whatever occurs placed above or beneath inthe order of the whole series, has been referred to one of them.Even the particular denominations by which beds are distin-guished, are of much importance in a geognostical description,however whimsical or improper may be their signification ortheir origin as taken from the language of miners. But whiletreating of the relative position of rocks on a surface of greatextent, it is indispensably necessary to consider the formationsor habitual associations of certain beds in a more general pointof view. It is then that discretion and circumspection are morenecessary in the distinction of rocks, and in their nomenclature.The work of M. Freiesleben on the Plains of Saxony, whichare upwards of 700 square leagues in extent (Geogr. Beschr. desKupferschiefergebirges, 1807—15), presents a beautiful modelof the union of local observations and geognostical generaliza-tions. These generalizations, these attempts to simplify the system|227| of formations, and to direct the attention more especially to greatcharacteristic features, should be more or less cautiously con-ducted, according as one describes the basin of a river, an iso-lated province, a great country such as France or Germany, oran entire continent.The more minute the investigation of districts becomes, themore does the connection between formations which appear atfirst perfectly independent, manifest itself by the great pheno-menon of alternation; that is to say, by a periodical successionof beds which present a certain analogy in their composition, andsometimes even in certain fossil organic bodies. It is thus that,in the transition-mountains, for example, in America (at the en-trance of the plains of Calabozo), beds of greenstone and eupho-tide, in Saxony (near Friedrichswalde and Meissen) the clay-slateswith glance-coal, the greywackes, porphyries, black limestones,and greenstones, constitute, from their frequent and repeatedalternation, a single formation. It often happens that subordi-nate beds appear only at the extreme limits of a formation, andassume the aspect of an independent formation. The cupreousand bituminous marls (Kupferschiefer), which occur in Thu-ringia between the alpine limestone (zechstein) and the red sand-stone (rothes liegende), and which have for ages been extensivelywrought, are represented in several parts of Mexico, of NewAndalusia, and of Southern Bavaria, by multiplied beds ofmarly clay, more or less carburetted, and included within thealpine limestone. Similar circumstances often give to gypsums,sandstones, and small beds of compact limestones, the appear-ance of particular formations. Their dependence on subordina-tion is known by their frequent association with other rocks, bytheir want of extent and of thickness, or by their total suppres-sion, which is frequently observed. It must not be forgot (andthis fact has struck me much in the two hemispheres) that thegreat formations of limestones, for example, the alpine lime-stone, have their sandstones, as the very generally extendedsandstones have their limestone beds. Thin beds of sandstones,of limestones, and of gypsums, characterize, in all the zones,the deposites of coal and rock-salt or muriatiferous clay (salzthon),isolated deposites, which are most commonly only covered bythese small local formations. It is by overlooking these consi-|228| derations, which should be familiar to every practical geologist,that the type of the great independent formations has been ren-dered too complicated.The phenomenon of alternation manifests itself, either locallyin rocks, superimposed several times upon each other, and con-stituting a single compound formation, or in the series of forma-tions considered in their aggregate. It is either greenstones andsyenites, slates and transition limestones, beds of limestones andof marl, that alternate immediately, or the whole is a system ofmica-slates, and of granular feldspathic rocks (granites, gneisses,and syenites), which reappear among the transition deposits,and which separate from the primitive homonymous system thegreywackes and limestones with orthoceratites. For the firstknowledge of this fact, one of the most important and leaststudied of modern geognosy, we are indebted to the beautifulobservations of Messrs Leopold von Buch, Brochant, and Hauss-mann. From the circumstance that, in the transition system,granular rocks, perfectly destitute of organic remains, succeedto compact rocks which contain these same remains, it has beenconcluded by geognosts of great name, that this alternation ofshelly and not-shelly rocks, might extend beyond the depositeswhich we call primitive. It has not been merely asked if theclay-slates, mica-slates, and gneisses, support the granites whichhave been considered as the oldest; the question has also beenagitated, whether greywackes and black limestones with madre-pores might not recur beneath those same granites. Accordingto this idea, the primitive and transition rocks would only forma single deposite; and the first might be regarded as intercalatedin a deposite posterior to the development of organic beings,and which might penetrate to an unknown depth into the inte-rior of the globe. I confess, that no direct observation can beas yet adduced in support of these opinions. The fragments ofrocks which I have seen contained in the lithoid lavas of the vol-canoes of Mexico, Quito, and Vesuvius, and which are thoughtto have been torn from the bowels of the earth, seem to belongto altered rocks of granite, mica-slate, syenite, and granularlimestone, and not to greywackes and limestone with madre-pores.We have preserved in the arrangement of rocks the greatdivisions known by the name of primitive, transition or inter-|229| mediary, secondary and tertiary deposites. The natural limitsof these four systems of rocks are the clay-slate with glance-coal or ampelite and lydian stone, alternating with compactlimestones and greywackes, the coal formation, and the forma-tions which immediately succeed the chalk. In geognosy, asin descriptive botany, the subdivisions or small groups offamilies have more distinct characters than the great divisionsor classes. It is the case with all the sciences; in which we risefrom individuals to species, from species to genera, and fromthese to still higher degrees of abstraction. A method neces-sarily rests upon differently graduated abstractions, and the pas-sages become more frequent in proportion as the characters aremore complex. The transition or intermediary formations ofWerner, which M. de Buch has first limited with the sagacityfor which he is distinguished (Moll’s Jahrb. 1798, b. ii. p. 254.),are connected by the ampelitic clay-slates, the syenites with zir-cons, the granite sometimes destitute of hornblende, and the an-thracitic mica-slate, with the primitive deposite; while the small-grained greywackes and madreporous and compact limestones,connect them with the coal sandstones and limestones of the se-condary deposites.Porphyries of very different formations have their principalseat among the transition rocks; but they break out, if we mayso speak, in considerable masses towards the secondary depo-sites, where they are connected with the coal sandstone, whilethey penetrate into the primitive class only as subordinate rocks,and of little thickness. The progressive motion, or, if I maybe allowed to use the expression, the extent of the oscilla-tion of the serpentine and euphotide, is very different. Thosediallage rocks, constituting many distinct formations, rarely co-vered with other rocks, stop short nearly at the lower boundaryof the secondary formations; towards the bottom they penetrateinto the primitive deposites to beyond the mica-schist. Thechalk seems to present a natural limit to the tertiary formations,which were first characterised by Messrs Cuvier and Brongniart,and justly, as deposites entirely different from the last secon-dary formations, described by the Freyberg School (Géogr.Miner. des Environs des Paris, p. 8. and 9.) Struck withthe relations which exist between the tertiary deposites and|230| the beds beneath the chalk, M. Brongniart has even recentlyproposed to designate the tertiary formations by the name ofupper secondary deposites, (Sur le gisement des Ophiolithes,p. 37). Compare also the very interesting geognostical discussionscontained in M. de Bonnard’s Traité des Roches, p. 138, 210,and 212.)The distinction of four deposites which we have successivelynamed, and of which three are posterior to the development oforganic life upon the globe, appears to me worthy of being re-tained, notwithstanding the passage of some formations to othersof a very different character, and notwithstanding the doubtswhich several very distinguished geognosts have founded uponthese passages. The classification of deposites marks greatepochs of nature; for example, the first appearance of some pe-lagic animals (zoophytes, cephalopodous mollusca), and the si-multaneous destruction of an enormous mass of monocotyle-dons. It presents as it were points of rest to the mind, and bykeeping in view that the formations themselves are much lessimportant than the great divisions, we have often an opportu-nity, on advancing from high mountains toward the plains, ofobserving the varied influence which the association of primi-tive and transition rocks, and that of secondary and tertiaryones, have exercised upon the inequality and configuration of theground. It is owing to this influence, that the aspect of thelandscape, the form of mountains and platforms, and the cha-racter of the vegetation, vary less, when we travel parallel to thedirection of the beds, than on cutting them at a right angle.(Greenough, Crit. Exam. of Geology, p. 38.)I continue, by following Messrs de Buch, Freiesleben, Bro-chant, Beudant, Buckland, Raumer (Geb. von Nieder-Schles.,1819), and other celebrated geognosts, to group the indepen-dent formations according to the divisions, into primitive, transi-tion, secondary, and tertiary deposites, without troubling myselfabout the impropriety of the greater number of these denomi-nations. I continue to separate the clay (with lignites) super-imposed upon the chalk, from that which is beneath it, and thechalk itself from the more ancient secondary formations. Butthese distinctions, by beds and groups of beds, so useful in thedescription of a deposite of small extent, ought not to prevent|231| the geognost, when he tries to rise to a more general point ofview, from connecting these clays and the chalk with the Juralimestone, and from regarding them as the last strata of thisgreat formation, composed of calcareous and marly beds.The inferior beds of the chalk (tuffeau) contain ammonites.The limestone of the mountain of St Peter of Maestricht indi-cates, as has already been observed by Messrs Omalius andBrongniart (Geogr. Miner., p. 13), the passage of the chalk toolder secondary limestones. Near Caen, according to thebeautiful observations of M. Prevost, the clays beneath the chalkcontain those same lignites which occur, in greater quantity, inthe clay which is situated immediately above the chalk: cerites,which bring to mind the coarse limestone of Paris, are seen in alimestone with trigonias, placed between clays inferior to thechalk and the oolitic beds. I do not insist upon these particu-lar facts; I mention them only to prove, by a striking example,how, on bringing together facts observed in different points ofthe same country, the great phenomenon of alternation revealsto us the connections between formations which at first sightappear to have nothing in common. It is the property of thosebeds which alternate with one another, of those rocks whichsucceed each other in periodical series, to present the most mark-ed contrast in the two beds which immediately follow eachother. In geognosy, as in the different parts of descriptive na-tural history, the advantage of classifications of variously gra-duated sections must be recognised, without losing sight of theunity of nature; and those who have contributed the most tothe advancement of natural philosophy, have possessed at onceboth the tendency to generalize, and the exact knowledge of amass of particular facts.It has been customary to terminate the series of deposites bythe volcanic rocks, and to make them succeed the secondary andtertiary, and even the alluvial deposites. In a system constructedupon the principle of relative antiquity, this arrangement seemsto me to have little to recommend it. It is without doubt thecase that lithoid lavas are spread over the most recent forma-tions, even over beds of gravel; nor can it be denied that thereexist volcanic productions of different epochs: but, from whatI have observed in the Cordilleras of Peru, of Quito, and of|232| Mexico, in a part of the world so celebrated for the frequencyof volcanoes, it seems to me, that the chief site of subterraneanfires is in the transition rocks, and beneath those rocks. I have ob-served, that all the burning or extinct craters of the Andes openin the midst of trap porphyries or trachytes, (Berl. Abhandl.der Kön. Acad., 1813, p. 131), and that these trachytes areconnected with the great transition porphyry, and syenite forma-tion. According to this observation, it appears more natural tome to make the secondary and volcanic deposites to follow thetransition deposite in a parallel manner, and as by bisection. Bythis new arrangement, the formation of porphyries, syenites andgreywackes, or that of transition porphyries, syenites and granites,occurs connected at the same time; 1st, With the porphyries ofthe red sandstone in the secondary coal-deposite; 2dly, With thetrachytes or trap porphyries which are destitute of quartz, andmixed with pyroxene. I employ with regret the term volcanicterrain, not that I doubt, like those who designate the trachytes,basalts and phonolites (porphyrschiefer), by the name of trapterrain, that all which I have associated in the volcanic terrainhas not been produced or altered by fire; but because severalrocks, intercalated between the (primitive?) transition and se-condary rocks, might also be volcanic. I would rather wish toavoid every (historical) idea of the origin of things, in a (sta-tistical) view of relative situation or superposition. At Skeen,in Norway, a basaltic and porous syenite, containing pyroxenes,is placed, according to the observation of M. de Buch, betweenthe transition limestone and the syenite with zircons. It is abed, not a dike; and this is a much less problematical pheno-menon than the basalt (Urgrünstein, Buch. Geogn. Beob. p.124, and Raumer, Granit des Riesengebirges, p. 70.) containedin the mica-schist of Krobsdorf in Silesia. The trachytes, withobsidian of Mexico, are intimately connected with the transitionporphyries which alternate with syenites. The amygdaloid be-longing to the red sandstone, assumes, on the Continent ofEurope, and in Equinoctial America, all the appearance of anamygdaloid of the basaltic formation. M. Boué, in his interest-ing Essai Geologique sur l’Ecosse, p. 126, 162, has describedpyroxenic rocks (dolerites) included in the red sandstone. With-out prejudging any thing regarding the origin of these masses,|233| or in general regarding that of all the primitive and transitionrocks, we designate here, by the name of Volcanic Terrains, theleast interrupted series of rocks altered by fire.In drawing up the enumeration of the different rocks, I havemade use of the names most generally employed by the geo-gnosts of France, Germany, England, and Italy: in attempt-ing to perfect the nomenclature of formations, I should be ap-prehensive of adding new difficulties to those which the discus-sion of relative positions already present. I have, however, care-fully avoided the denominations, too long preserved, of underand upper limestone; of gypsum of the first, second, or thirdformation, of old and new red sandstone, &c. These denomina-tions without doubt present a true geognostical character; theyhave relation, not to the composition of rocks, but to their rela-tive age. However, as the general type of the formations ofEurope cannot be modelled after that of a single district, thenecessity of admitting parallel formations (sich vertretende Ge-birgsarten), renders the names of first or second gypsum, of oldor middle sandstone, extremely vague and obscure. In onecountry it is proper to consider a bed of gypsum or of commonsandstone as a particular formation; while in another, it shouldbe regarded as subordinate to neighbouring formations. Thegeographical denominations are without doubt the best; theygive rise to precise ideas of superposition. When it is said thata formation is identical with the porphyry of Christiania, thelias of Dorsetshire, the sandstone of Nebra (bunter sandstein),the coarse limestone of Paris, these assertions leave no doubtin the mind of an experienced geognost, regarding the positionwhich is to be assigned to the formation in question. It is alsoby silent convention, as it were, that the words zechstein ofThuringia, Derbyshire Limestone, Paris Formation, &c. havebeen introduced into mineralogical language. They express alimestone which immediately succeeds the red sandstone of thecoal deposite, a transition limestone placed beneath the coal sand-stone, and lastly formations of more recent origin than the chalk.The only difficulties which the multiplicity of these geographicaldenominations presents, consist in the choice of names, and inthe degree of certainty which may have been acquired, regard-ing the position or relative age of the rock to which the others|234| are referred. The English geognosts look upon the continentfor their lias and red marl; the German for their bunte sand-stein and muschelkalk. These words present themselves in theminds of travellers associated with remembrances of localities. Itis not of so much importance, therefore, to produce precise ideas,as to make choice of localities generally known, and which arecelebrated, either by the working of mines, or by descriptiveworks.In order to diminish the effects of national vanity, and to at-tach new names to more important objects, I proposed a longtime ago, (1795), the denominations of Alpine Limestone, andJura Limestone. A portion of the High Alps of Switzerland,and the greater part of Jura, are without doubt formed of thesetwo rocks: the names, however, generally received at the presentday, of Alpine Limestone (Zechstein), and Jura Limestone,should in my opinion be modified or entirely abandoned. Thelower beds of the Jura mountains, filled with gryphites, belongto an older formation, perhaps to the zechstein; and a greatpart of the limestone of the Alps of Switzerland assuredly isnot zechstein; but, according to Messrs de Buch and Escher,transition limestone. It would therefore be better to choose thegeographical names of rocks from among the names of isolatedmountains, the whole visible mass of which belongs only to asingle formation, than to derive them, as I have erroneouslydone, from entire chains. I have thought, and many geognostshave formed the same opinion, that the Jura limestone (cavern-ous limestone of Franconia) was generally placed upon the con-tinent, beneath the Nebra sandstone, (bunte sandstein), betweenthis sandstone and the zechstein. Subsequent observationshave proved, that the name of Jura Limestone had with reasonbeen applied to rocks which are very distant from the moun-tains of Western Switzerland; but that the true geognosticalplace of this formation, (when there is not a suppression of theinferior formations), occurs above the Nebra sandstone, betweenthe shell-limestone (muschelkalk, or the quadersandstein), andthe chalk. A geographical name, justly applied to several ana-logous rocks, renders us attentive to their identity of relativeposition; but the place which homonymous rocks ought to oc-cupy in the total series, is not well determined except when the|235| geographical name has been selected, after having acquired aperfect certainty regarding their position. Circumstances arethe same with regard to the relative age of the molasse of Ar-govia (nagelfluhe) and of the Pirna quadersandstein (grès blancof M. de Bonnard), two rocks of very recent origin, which havebeen very well examined separately, but whose relations to eachother, and to the chalk and Jura limestone, have only been illus-trated of late. One may therefore be pretty sure of having metin the New Continent with rocks identical with the molasse orquadersandstein, without being able to pronounce with certain-ty upon their relations with all the secondary or tertiary rocks.When rocks are not immediately in contact, and are not coveredby deposites of known position, their relative age can only beconjectured from simple analogies.The terms of the geognostical series are either simple or com-plex. To the simple terms belong the greater number of theprimitive formations: the granites, gneisses, mica-slates, clay-slates, &c. The complex terms occur in greater numbersamong the transition rocks: there, each formation includes anentire group of rocks, which alternate periodically. The termsof the series are not transition limestones or greywackes, consti-tuting independent formations; they are associations of clay-slate, greenstone, and greywacke; of porphyry and greywacke;of granular steatitic limestone, and of conglomerates, composedof primitive rocks; of clay-slate and black limestone. Whenthese associations are formed of three or four rocks which alter-nate, it is difficult to give them significant names,—names indi-cative of the whole composition of the group,—of all the partialmembers of the complex term of the series. It may then assistin fixing the groups in the memory, to retrace the rocks whichpredominate in them, without being absolutely wanting in theneighbouring groups. It is in this manner that the granularsteatitic limestone characterises the Tarantaise formation; thegreywacke, the great transition formation of the Hartz and ofthe banks of the Rhine; the metalliferous porphyries rich inhornblende, and almost destitute of quartz, the formation ofMexico and of Hungary. If these phenomena of alternationattain their maximum in the transition districts, still they are notentirely excluded from the primitive and secondary terrain. In|236| both of these terrains, complex terms are mixed with the simpleterms of the geognostical series. I shall mention among the se-condary formations, the sandstone placed below the alpine lime-stone, (the Nebra sandstone, bunte sandstein), which is an asso-ciation of marly clay, sandstone and oolites; the limestone whichcovers the red-sandstone of the coal-formation (the zechstein oralpen-kalkstein), which is a less constant association of limestone,of (muriatiferous) gypsum, of stinkstone and of pulverulentbituminous marl. In the primitive class we find the threefirst terms of the series; the oldest rocks either isolated, or al-ternating two and two, according as they are geognosticallymore approximated by their relative age, or the whole three al-ternating. The granite sometimes forms constant associations withthe gneiss, and the gneiss with the mica-slate. These alterna-tions follow particular laws: we see, (for example in Brazil, and,although less distinctly, in the maritime chain of Venezuela), thegranite, gneiss and mica schist in a triple association; but I havenot found granite alternating alone with mica-slate, or gneiss andmica-slate alternating by themselves with clay-slate.We must not confound, and on this point I have often in-sisted in the present article, rocks passing insensibly to thosewhich are in immediate contact with them; for example, mica-slates, which oscillate between gneiss and clay-slate, with rockswhich alternate with one another, and which preserve all theirdistinctive characters of composition and of structure. M.D’Aubuisson has long ago shewn how chemical analysis approxi-mates the clay-slate to mica. (Journal de Physique, vol. 68.p. 128.; Traité de Geognosie, vol. ii. p. 97.) The first, it istrue, has not the metallic lustre of mica-slate; it contains a lit-tle less potash, and more carbon; the silex does not unite intonodules or thin laminæ of quartz, as in the mica-slate; but itcannot be doubted, that scales of mica form the principal baseof the clay-slate. These scales are so joined together, that theeye cannot distinguish them in the mass. It is perhaps thissame affinity which prevents the alternation of clay-slates andmica-slates: for in these alternations Nature seems to favour theassociation of heterogeneous rocks; or, to make use of a figura-tive expression, she delights in the associations whose alternatingrocks present a great contrast of crystallization, of mixture and|237| of colour. At Mexico, I have seen dark greenstones, al-ternate thousands of times with reddish-white syenites, abound-ing more in quartz than in felspar. In this greenstone therewere veins of syenite, and in the syenite veins of greenstone;but none of the two rocks passed into the other. (Essai poli-tique sur la Nouvelle Espagne, v. ii. p. 523.) They present atthe limit of their mutual contact, differences as marked as theporphyries which alternate with the greywackes or with the sye-nites, as the black limestones which alternate with the transitionclay-slates, and so many other rocks of entirely heterogeneouscomposition and aspect. Further, when, in primitive deposites,rocks more related by the nature of their composition than bytheir structure or mode of aggregation, for example, the granitesand gneisses, or the gneisses and mica-schists, alternate; theserocks do not by any means show the same tendency to pass in-to each other, as they present, when isolated in formations whichare not of a complex character. We have already observed, thatoften a bed β, becoming more frequent in the rock α, announcesto the traveller that the simple formation α, is to be succeededby a compound formation, in which α and β alternate. Fartheron, it happens, that β assumes a greater development; that α isno longer an alternating rock, but a simple bed subordinate to β,and that this rock β shows itself alone, until, by the frequentrepetition of beds γ, it becomes the precursor of a compound for-mation of β, alternating with γ. We might substitute for thesesigns the words granite, gneiss, and mica-slate; those of por-phyry, greywacke and syenite; of gypsum, marl and fetid lime-stone (stinkstein). Pasigraphic language has the advantage ofgeneralizing the problems; it is more conformable to the wantsof geognostical philosophy, of which I attempt to present herethe first elements, in so far as they have relation to the study ofthe superposition of rocks. Now, if often between formationswhich are simple and very closely allied, in the order of theirrelative antiquity, between the formations α, β, γ, there occurcompound formations interposed, αβ and βγ, (that is to say α al-ternating with β, and β alternating with γ); we observe, also,although less frequently, that a formation (for example α,) as-sumes so extraordinary an increase, that it envelopes the forma-|238| tion β; and that β, instead of showing itself as an indepen-dent rock, placed between α and γ, is now nothing but a bed inα. It is thus, that, in Lower Silesia, the red-sandstone containsthe formation of zechstein; for the limestone of Runzendorf,filled with impressions of fishes, and analogous to the bitumi-nous marl abounding in fishes of Thuringia, is entirely de-veloped in the coal-formation. (Buch, Beob. vol. i. p. 104. 157.;Id. Reise nach Norwegen, vol. i. p. 158.; Raumer, Gebirge vonNieder-schlesien, p. 79.) M. Beudant, Voy. Miner., vol. iii.p. 183., has observed a similar phenomenon in Hungary. Inother districts, for example, in Switzerland, at the southern ex-tremity of Saxony, the red-sandstone disappears entirely; be-cause it is replaced, and, so to speak, overcome, by a prodigiousdevelopment of greywacke or of alpine limestone. (Freiesleben,Kupfersch. p. 109.) These effects of the alternation or unequaldevelopment of rocks, are so much the more worthy of attention,that their study may throw light upon some apparent deviationsfrom a generally acknowledged type of superposition, and thatit may serve to refer to a common type the series of position ob-served in very distant countries.In order to designate the formations composed of two rockswhich alternate with another, I have generally preferred thewords granite and gneiss, syenite and greenstone, to themore commonly adopted expressions of granite-gneiss, syenite-greenstone. I was apprehensive that this last method of desig-nating formations composed of alternating rocks, might rathergive rise to the idea of a passage from granite to gneiss, fromsyenite to greenstone. In fact, a geognost, whose works uponthe trachytes of Germany have not been sufficiently appreciated,M. Nose, has already made use of the words granite-porphyriesand porphyry-granites, to indicate varieties of structure and as-pect, to separate the porphyritic granites from porphyries,which, from the frequency of crystals imbedded in the mass,presents an aggregational, a true granitic structure. By adopt-ing the denominations of granite and gneiss, of syenite and por-phyry, of greywacke and porphyry, of limestone and clay-slate,no doubt is left regarding the nature of the complex terms ofthe geognostical series *.

* Translated from Essai Geognostique par Baron Alexandre de Humboldt.|239| M. Humboldt next proceeds to consider the natural history offossil organic remains, as connected with formations. This veryinteresting paper on petrifactions, has already appeared in No.xvii. of our Journal.

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