Digitalphilologie

Titel

On Rock Formations

Jahr

1824

Ort

Boston, Massachusetts

Nachweis

in: The Boston Journal of Philosophy and the Arts, intended to exhibit a view of the progress of discovery in natural philosophy, mechanics, chemistry, geology and mineralogy, natural history, comparative anatomy and physiology, geography, statistics, and the fine and useful arts 2:1 (Juli 1824–Juli 1825), S. 15–27; 2:2 (Juli 1824–Juli 1825), S. [105]–118.

Sprache

Englisch

Typografischer Befund

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

Identifikation

Textnummer Druckausgabe: IV.26
Dateiname: 1822-Independance_des_formations-3

Statistiken

Seitenanzahl:	27
Zeichenanzahl:	66782

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)

|15|

On Rock Formations. By Baron AlexanderHumboldt. *

The word formation designates, in Geognosy, either themanner in which a rock has been produced, or an assem-blage of mineral masses, which are so connected together,that they are supposed to have been formed at the sameperiod, and present, in the most distant parts of the world,the same general relations of situation and position. It isthus that the formation of obsidian and basalt is attributed tosubterranean fires; and thus also that we say the formationof transition clay-slate contains lydian-stone, chiastolite, alum-slate, and alternating beds of black limestone and porphyry.The first acceptation of the word is better adapted to thegenius of the language; but it has relation to the origin ofthings, to an uncertain science founded upon geogonic hypo-thesis. The second acceptation, now generally adoptedby the French mineralogists, has been borrowed from thecelebrated school of Werner: it indicates what is, not whatis 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 generalideas. Rocks which alternate with one another, which areusually associated, and which present the same relations ofposition, constitute a formation; the union of several forma-

* Translated from Essai Géognostique par Alexandre de Humboldt.|16| tions constitutes a district or terrain; but these different termsof rocks, Formation and Terrain, are employed as synony-mous in many works of geognosy.The diversity of the rocks, and the relative disposition ofthe beds which form the oxidised crust of the earth, have,from the most remote times, fixed the attention of men.Wherever the working of a mine was directed upon a de-posit of salt, of coal, or of clay-iron, which was covered witha great number of beds of different natures, it gave rise toideas more or less precise regarding the system of rocks pe-culiar to a district of small extent. Furnished with theselocal details, and full of prejudices which arise from custom,the miners of a country would disperse themselves over theneighbouring districts. They would do what geognosts haveoften done in our days; they would judge of the positionof rocks of whose nature they were ignorant, according toimperfect analogies, according to the circumscribed ideaswhich 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 dictricts, 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 consti-tution. This very old popular opinion has been adopted andsupported, in different countries, by very distinguished men;but since geognosy has been elevated to the rank of a science,the art of interrogating nature brought to perfection, andjourneys made into distant countries, have presented a moreexact comparison of different districts, great and immutablelaws have been discovered in the structure of the globe, andin the superposition of rocks. Since, then, the most strikinganalogies of situation, of composition, and of organic bodiescontained in contemporaneous beds, have manifested them-selves in the two worlds, in proportion as we become accus-tomed to consider the formations under a more generalpoint of view, even their identity becomes every day moreprobable.In fact, on examining the solid mass of our planet, we per-ceive that some of those substances with which oryctognosy(descriptive mineralogy), makes us acquainted in their indi-vidual capacities, are met with in constant associations, andthat these associations, which are designed by the name ofCompound Rocks, do not vary like organic beings, accord-|17| ing to the differences of the latitudes, or of the isothermallines in which they occur. The geognosts who have travel-led over the most remote countries, have not only met in thetwo hemispheres with the same simple substances, quartz,felspar, mica, garnet or hornblende; but they have also foundthat the great mountain masses present almost every wherethe same rocks, that is to say, the same assemblages of mica,quartz, and felspar in the granite; of mica, quartz, and gar-nets in the mica-slate; of felspar and hornblende in thesyenite. If it has sometimes been thought at first that arock belonged exclusively to a single portion of the globe, ithas 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 tothem, (Humboldt, Geographie des Plantes, 1807, p. 115; Vuesdes Cordilleres, vol. i. p. 122). Rocks are found to be iden-tical where organic beings have undergone the most variedmodifications.But this identity of composition, this analogy which is ob-served in the association of certain simple mineral substances,might be independent of the analogy of relative situation andof superposition. One may have brought from the islands ofthe Pacific Ocean, or from the Cordilleras of the Andes, thesame rocks which are observed in Europe, without his beingpermitted to conclude that these rocks are superimposed inthe same manner, and that after the discovery of one of themit might be predicted with some degree of certainty what arethe other rocks which occur in the same places. It is to dis-cover these analogies of situation and relative position, thatthe labours of geognosts should tend, who delight to investi-gate the laws of inorganic nature. In the following tables,we have attempted to unite all that is known with certainty,regarding the superposition of rocks in the two Continents,to the north and south of the Equator. These types of forma-tions will not only be extended, but also variously modified,in proportion as the number of travellers qualified to makegeognostical observations shall become increased, and ascomplete monographs of different districts at great distancesfrom each other shall furnish more precise results.The exposition of the laws observed in the superposition ofrocks, forms the most solid part of the science of geognosy.It must not be denied, that the observations of geognosticalsituation often present great difficulties, when the point ofcontact of two neighbouring formations cannot be reached,|18| or when they do not present a regular stratification, or whentheir relative situation is not uniform, that is to say, when thestrata of the upper deposits are not parallel to the strata ofthe lower. But these difficulties (and this is one of the greatadvantages of observations which embrace a considerablepart of our planet), diminish in number, or disappear entirely,on comparing several districts of great extent. The super-position and relative age of rocks, are facts susceptible ofbeing established immediately, like the structure of the organsof a vegetable, like the proportions of elements in chemicalanalysis, or like the elevation of a mountain above the levelof the sea. True geognosy makes known the outer crust ofthe globe, such as it exists at the present day. It is a scienceas capable of certainty as any of the physical descriptive sci-ences can be. On the other hand, all that relates to the ancientstate of our planet, to those fluids which, it is said, held allthe mineral substances in a state of revolution, to those seaswhich we have raised to the summit of the Cordilleras, tomake them again disappear, is as uncertain as are the forma-tion of the atmosphere of planets, the migrations of vegetables,and the origin of different varieties of our species. Yet theperiod is not very remote when geologists occupied themselvesby preference with the solution of these almost impossibleproblems, 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 observationsfurnished by the practical examination of different districts.We shall 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 inone country, sometimes appear closely connected in another.What announces the independence of a formation, as hasbeen very justly observed by M. de Buch, is its immediatesuperposition upon rocks of a different nature, and whichconsequently ought to be considered as more ancient. Thered sandstone is an independent formation, because it is super-imposed indifferently upon black (transition) limestone, uponmica-slate, or upon primitive granites; but in a country wherethe great formation of syenite and porphyry predominates,these two rocks constantly alternate. There results that thesyenite rock is dependent upon the porphyry, and scarcelyany where covers by itself the transition clay-slate or primi-tive gneiss. The independence of formations does not, be-|19| sides, by any means exclude the uniformity or concordance ofposition; it rather excludes the oryctognostic passage of twosuperimposed formations. The transition districts have veryoften the same direction and the same inclination as the primi-tive ones; and yet, whatever approximation there may bebetween their origin, we are not the less warranted to con-sider the anthracitic mica-slate or the grey-wacke, alternatingwith porphyry, as two formations independent of the primi-tive 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 theright which one has of regarding a rock as a distinct forma-tion. It is because the independent formations are placedindifferently on all the older rocks, (the chalk upon thegranite, the red sandstone upon the primitive mica-slate), thatthe assemblage of a great number of observations made uponvery distant points, becomes eminently useful in the deter-mination of the relative age of rocks. In order to determinethat the zircon-sienite is a transition rock, it must have beenseen resting upon formations posterior to the black lime-stone with orthoceratites. Observations made upon the por-phyries and sienites of Hungary by M. Beudant, one of themost distinguished geologists of the present times, may throwmuch light upon the formations of the Mexican Andes. Itis thus that a new vegetable discovered in India, elucidatedthe natural affinity between two families of plants belongingto Equinoctial America.The order which has been followed in the table of forma-tions, is that of the situation and relative position of rocks.I do not pretend that this position is observed in all thecountries of the globe; I merely point it out such as it hasappeared the most probable, after the comparison of a greatnumber of facts which I have collected. It is by the idea ofthe relative age of formation, that I have been guided in thiswork, imperfect as it still is. I had begun it long before myjourney to the Cordilleras of the New Continent, from theyear 1792, when, on leaving the Freyberg School, I was ap-pointed to the direction of the Mines in the mountains of theFichtelgebirge. The same rock may vary in composition,integrant parts may have been abstracted, and new sub-stances may occur disseminated, without the rock’s changingits denomination in the eyes of the geognost who is engagedwith the superposition of formations. Under the equator, asin the north of Europe, strata of a true transition syenite losetheir hornblende, without the mass becoming another rock.|20| The granites of the banks of the Orinoco sometimes assumehornblende as an integral part, and yet do not cease to beprimitive granite, although this may not be of the first oroldest formation. These facts have been observed by allpractical geologists. The essential character of the identityof an independent formation is its relative position, the placewhich it occupies in the general series of formations. (Seethe classical Memoir of M. de Buch, Ueber den Begriff einer,in the Mag. der Naturf., 1810, p. 128—133.) It is on thisaccount that an isolated fragment, a specimen of rock foundin a collection, cannot be determined geognostically, that isto say, it cannot be referred with certainty to a particularformation, constituting one of the numerous beds of which thecrust of our planet is composed. The presence of chiastolite,the accumulation of carbon or nodules of compact limestonein the clay-slates, nigrine and epidote in the syenites, (alter-nating with granite and porphyries), conglomerates or pud-ding-stones, having a basis of anthracitic mica-slate, may,without doubt, be characteristic of transition formations; inthe same way as, according to the useful labours of M.Brongniart, petrifactions of shells, in a good state of pre-servation, sometimes indicate directly such or such a bed oftertiary deposits. But these cases, where we are guided bydisseminated substances or by characters purely geological,comprehend but a small number of rocks of a recent origin,and observations of this kind often lead only to negative facts.The characters taken from the colour, from the grain, andfrom small veins of carbonate of lime, which traverse calca-reous rocks; those which are furnished by the fissility andsilky lustre of clay-slate, the aspect and undulations more orless marked of the scales of mica in mica-slate; and, lastly,the size and colouring of the crystals of felspar in the granitesof different formations, may, like all that is connected simplywith the physiognomy of minerals, lead the most expert ob-server into error. The white and black tints undoubtedlyin most instances distinguish the primitive and transition lime-stones; the Jura formation, especially in its upper beds, isalso without doubt generally divided into thin whitish beds,having a dull, even, or conchoidal fracture, with very flatcavities: but in the mountains of transition limestone thereare isolated masses which, in colour and texture, resemblethe oryctognostic characters of the Jura limestone; and tothe south of the Alps there are hills belonging to tertiarydeposits, where we find rocks analogous to the slaty and dullJura limestone (in as far as regards appearance), in formations|21| placed above the chalk, and which resemble the limestoneused for lithographic purposes. Were names taken fromtheir oryctognostical characters alone to be preferred in dis-tinguishing formations, the different strata of the same com-pound rock having a considerable thickness, and extended toa great length in a particular direction, would often seem tobelong to different rocks, according to the points at whichspecimens were taken. Consequently we can only determinegeognostically in collections, suites of rocks of which the mu-tual superposition is known.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 under-valuing the eminent services which the most rigorous oryc-tognostic examination, the minute investigation of the com-position of rocks, have rendered to modern geognosy, andespecially to the knowledge of the relative position of forma-tions. Although, according to the discoveries of M. Haüy,regarding the intimate nature of inorganic and crystallizedsubstances, there does not exist, properly speaking, a passageor transition of one mineral species to another; (Cordier, surles Roches volcan., p. 33, and Berzelius, Nouv. Syst. de Mineral,p. 119) the passages of masses or pastes of rocks, are not limitedto formations which are commonly distinguished by the nameof Compound Rocks. Those which are thought simple, forexample, the transition or secondary limestones, are partlyamorphous varieties of mineral species, of which there existsa crystallized type, partly of aggregates of clay, carbon, &c.,which cannot be submitted to any fixed determination. It isupon the variable proportions of these heterogeneous mix-tures, that the passage of marly limestones to other schistoseformations is founded. (Haüy, Tableau comparatif de la Cris-tallographie, p. 27—30). All the amorphous pastes of rocks,however homogenous they appear at first sight, the bases ofporphyries and euphotides (serpentines), as well as thoseproblematical black masses which constitute the basanite(basalt) of the ancients, and which are not all greenstonessurcharged with hornblende, are susceptible of being sub-jected to mechanical analysis. M. Cordier has applied thisanalysis in an ingenious manner to the diabases, dolerites,and other more recent volcanic productions. The most ap-parently minute oryctognostic examination, cannot be indif-ferent to the geognost who examines the age of formations.It is by this examination that we are enabled to form a justidea of the progressive manner in which, by internal develop-|22| ment, that is to say, by a very gradual change in the proportionsof the elements of the mass, the passage is made from onerock to a neighbouring. The transition slates, whose struc-ture appears at first so different from that of the granites orporphyries, present to the attentive observer striking exam-ples of insensible passages to granular rocks of porphyriticor 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, becomesdeveloped, and separates into distinct and clearly crystallizedspangles; at the same time, the felspar and quartz becomevisible; and the mass assumes a granular aspect, with veryelongated grains: this is a true transition gneiss. By degrees,the grains lose their common direction; the crystals arrangethemselves around many centres; the rock becomes a transi-tion granite or syenite. In other cases, the quartz alone isdeveloped; it augments, and becomes rounded into nodules,and the slate passes to the best characterized grey-wackes.By these certain signs, geognosts, to whom the appearances ofnature have become familiar by long examination, becomeaware beforehand of the proximity of granular, granitic, andarenaceous rocks. Analogous passages of primitive mica-slate to a porphyritic rock, and the return of this rock togneiss, are observed in the eastern parts of Switzerland.(See the luminous developments given by M. de Raumer,Fragments, p. 10, and 47; M. Leopold de Buch, in his Voyagede Glaris à Chiavenna, fait en 1803, and inserted in the Magaz.der Berl. Naturf., vol. iii. p. 115). But these passages are notalways insensible and progressive; the rocks often also suc-ceed each other quickly, and in a very abrupt manner; often(for example, at Mexico, between Guanaxuato and Ovexeras),the limits between the slates, the porphyries and syenites, areas distinct as the limits between the porphyries and lime-stones; but even in this case, geognostical relations with thesuperimposed 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 some-times pass 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,and washings, (an analysis of which, M. Fleuriau de Bellevue|23| made the first attempt, that was crowned with success: Journ.de Physique, vol. li. p. 162), throws light at once, 1st, Uponthe large crystals which are isolated and separated from themicroscopic crystals entangled in the mass; 2dly, On themutual passages of some rocks, superimposed the one uponthe other; 3dly, On the subordinate beds, which are of thesame nature as one of the elements of the amorphous mass.All these phenomena are produced, if we may so speak, byinternal development; by variation in the constituent partsof a heterogeneous mass. Crystalline molecules, invisibleto the eye, occur enlarged and disengaged from the compacttissue of the paste; by their assemblage and mixture withnew substances, they insensibly become intercalated beds ofconsiderable thickness; and not unfrequently they even be-come new rocks.It is the intercalated beds which especially merit thegreatest attention. (Leonhard, Kopp, and Gærtner, Propæd.der Miner., p. 158.) When two formations succeed each otherimmediately, it happens that the beds of the one begin at firstto alternate with the beds of the other, until (after these pre-cursors of a great change) the newest formation shows itselfwithout any mixture of subordinate beds. (Buch, Geogn.Beob. vol. i. p. 104, 156; Humboldt, Rel. Hist. vol. ii. p. 140).The progressive developments of the elements of a rock, may,consequently, have a great degree of influence upon the rela-tive position of the mineral masses. Their effects belong tothe province of geology; but, in order to discover andappreciate them, the observer must call to his assistanceoryctognosy.In exposing the intimate relations by which we often seethe phenomena of composition connected with those of rela-tive situation, it has not been my intention to speak of thepurely oryctognostic method, which considers rocks accord-ing to the analogy of their composition alone. (Journal desMines, vol. xxxiv., No. 199). In the classifications of thismethod, abstraction is made of every idea of superposition;but they do not the less give rise to interesting observationsregarding the constant assemblage of certain minerals. Apurely oryctognostic classification, multiplies the names ofrocks more than is required by geognosy, when occupiedwith superposition alone. According to the changes whichthe mixed rocks undergo, a stratum of great extent andthickness may contain (we must repeat it here) parts towhich the oryctognost, who classes rocks according to theircomposition, would give entirely different denominations.|24| These remarks have not escaped the learned author of theClassification Mineralogique des Roches; they must have pre-sented themselves to an experienced geognost, who has sosuccessfully investigated the superposition of the deposits ofwhich he has treated. “We must not confound,” says M.Brongniart, in his late Memoir on the position of the Ophi-olites, “the relative positions, the orders of superposition ofthe deposites and of the rocks which compose them, withpurely mineralogical descriptions. The neglect of makingthe proper distinction in this case, would necessarily be pro-ductive of confusion in the science, and would retard itsprogress.” The arrangement which we give at the end ofthis article, is by no means what is called a classification ofrocks; there will not even be found united, under the titleof particular sections (as in the old geognostical method ofWerner, or in the excellent Traité de Geognosie of M. D’Au-buisson), all the primitive formations of granite, nor all thesecondary formations of sandstone and limestone. It hasbeen attempted, on the contrary, to place each rock as itoccurs in nature, according to the order of its superpositionor of its respective age. The different formations of graniteare separated by gneisses, mica-slates, black-limestones, andgrey-wackes. In the transition rocks, we have separatedthe formations of porphyries and syenites of Mexico andPeru, which are anterior to the grey-wacke, and to the lime-stone with orthoceratites, from the much more recent forma-tion of the zircon-porphyries and syenites of Scandinavia.In the secondary rocks, we have separated the oolitic sand-stone of Nebra, which is posterior to the alpine limestone orzechstein, from the red-sandstone, which belongs to the sameformation with the secondary porphyry and amygdaloid.According to the principle which we follow, the same namesof rocks occur several times in the same table. Anthraciticmica-slate is separated, by a great number of older forma-tions, from the mica-slate anterior to the primitive clay-slate.Instead of a classification of granitic, schistose, calcareousand arenaceous rocks, it has been my object to present asketch of the geognostical structure of the globe; a table inwhich the superimposed rocks succeed each other, from be-law upwards, as in those ideal sections which I designed in1804, for the benefit of the Mexican School of Mines, and ofwhich many copies have been distributed since my return toEurope. (Bosquejo de una Pasigrafia geognostica, con tablasque ensenan la estratificacion y el parallelismo de las rocas enambos Continentes, para el uso del Real Seminario de Mineria de|25| Mexico). These Pasigraphic tables united to my own obser-vations made in both Americas what had at that period beenknown with precision regarding the relative position of theprimitive, intermediary and secondary rocks in the OldContinent. They presented, together with the type whichmight be considered as the most general, the secondarytypes, that is to say, the beds which I have named parallel.This same method has been followed in the work which Inow publish. My parallel formations are geognostical equiva-lents; they are rocks which represent each other. (Seethe Traité de Geologie de M. d’Aubuisson, vol. ii. p. 255).In England, and on the opposite Continent, there does notexist an identity of all the formations: there exist equivalentsor parallel formations. That of our coal situated betweenthe transition masses and the red-sandstone, the position ofthe rock-salt which occurs on the Continent in the alpinelimestone, and the position of our oolites in the Nebra sand-stone and Jura limestone, may guide the geognost in theapproximation of remote formations. In England, we ob-serve the coals placed upon transition formations; for exam-ple, upon the mountain-limestone of Derbyshire and of SouthWales, and upon the transition sandstone, or old red-sandstoneof Herefordshire. I have thought that I recognized in themagnesian-limestone the red marl, the lias and white oolitesof Bath, the united formations of the alpine limestone, of theoolitic sandstone and Jura limestone. In comparing the for-mations 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 andof Thuringia, we must not think of opposing to each individualrock a parallel one; it must be recollected, that a single for-mation may represent several others. It is according to thisprinciple that beds of clay, lying beneath the chalk, may, inFrance, be separated in the most distinct manner from theoolitic limestone beds: while in Switzerland, in Germany,and in South America, they have for equivalents beds of marlssubordinate to the Jura limestone. The gypsums, which, inone district, are sometimes only intercalated beds in thealpine limestone or oolitic sandstone, in another district, as-sume all the appearance of independent formations, andoccur interposed between the alpine limestone and the ooliticsandstone, between this sandstone and the muschelkalk. Thelearned Oxford Professor, Mr. Buckland, whose extensiveresearches have been equally useful to the geognosts ofEngland and of the Continent, has lately published a table|26| of parallel formations, or, as he calls them, equivalents ofrocks, which only extends from the 44th to the 54th degreeof north latitude, but which merits the greatest attention.(On the Structure of the Alps, and their relation with the rocks ofEngland, 1821.)As in the history of ancient nations, it is easier to verifythe series of events in each country, than to determine theirmutual coincidence; so also more accuracy can be attainedin estimating the superposition of formations in isolated re-gions, than in determining the relative age or parallelism offormations which belong to different systems of rocks. Evenin countries which are not widely separated, in France, inSwitzerland, and in Germany, it is not easy to fix the relativeantiquity of the muschelkalk, of the molasse of Argovie, andof the quadersandstein of the Hartz; because rocks of gen-eral occurrence are here most commonly wanting, which,according to the happy expression of M. de Grüner, serve asa geognostical horizon, and with which we might compare thethree formations in question. When rocks are not in imme-diate contact, we can only judge of their parallelism by therelations of age existing between them and other formationsby which they are united.These inquiries of comparative geognosy, will long occupythe sagacity of observers; and it is not surprising that thosewho set out with the idea of retracing each formation in allthe individuality of its relative position, interior structure,and subordinate beds, should finish with utterly denying allanalogy of superposition. I had the advantage of visiting,previous to my journey to the Equator, a great part of Ger-many, of France, of Switzerland, of England, of Italy, ofPoland, and of Spain. During these excursions, my attentionwas particularly directed to the relative position of forma-tions, a phenomenon which I calculated upon discussing in aspecial work. On my arrival in South America, and while atfirst traversing in different directions the vast deposites whichstretch from the maritime chain of Venezuela to the basin ofthe Amazon, I was singularly struck with the conformity ofposition which the two Continents present. (See my firstsketch of a Geological Table of Equinoctial America, in theJournal de Phys., vol. liii. p. 38). Subsequent observations,which included the Cordilleras of Mexico, of New Grenada,of Quito, and of Peru, from the 21st degree of north latitudeto the 12th degree of south latitude, have confirmed thesefirst perceptions. But in speaking of analogies which areobserved in the relative position of rocks, and of the unifor-|27| mity of those laws which reveal to us the order of Nature, Imight adduce a testimony otherwise of more weight thanmine, that of the great geognost whose works have thrownthe greatest light upon the structure of our globe. M. Leo-pold de Buch has pushed his researches from the Archipelagoof the Canary Isles to beyond the Polar Circle to the 71st de-gree of latitude. He has discovered new formations situatedbetween others already known; and, in the primitive as inthe transition deposites, in the secondary as well as in thevolcanic, he has been struck with the great features by whichthe table of formations is characterized in the most distantregions.(To be continued.)

|105|

On Rock Formations. By Baron Alexander Hum-boldt. [Edin. Jour.][Concluded from page 21.]

From that scepticism which would deny the existence ofany kind of regular order in the position of rocks, it is properto distinguish an opinion which has sometimes found supportersamong experienced observers. According to this opinion, theformations 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 isthought that a secondary rock may have been formed on onespot of the globe, while transition rocks did not yet exist onanother spot. In this supposition, no allusion is had to thosegranitic rocks which are found lying above limestone con-taining orthoceratites, and which are consequently newerthan the primitive rocks. It is a fact generally admitted atthis day, that formations of analogous composition have beenrepeatedly deposited at epochs far removed from each other.The doubt which we are now considering, (though we do notpartake in it), bears on a point much less clearly established,—the ascertaining whether certain mica-slate rocks, evidentlysituated in the midst of a country of primitive rocks, and|106| placed below those in which the vestiges of organic life beginto appear, are newer than the secondary rocks of anothercountry. I confess, that, in the part of the globe which I havehad an opportunity of examining, I have not seen any thingthat might tend to confirm this opinion. Granular syeniticrocks repeated twice, perhaps even three times, in primitiveintermediary (and secondary) deposits, are analogous phe-nomena with which we have become acquainted within thesefifteen years. This disagreement in regard to age of greathomonymous formations, does not by any means seem to meto be proved as yet by direct observations, made upon thecontact of superimposed formations. The chalk or Jura lime-stone may, on one hand, immediately cover primitive granite,and, on the other, be separated from it by numerous secondaryand transition rocks: these very common facts demonstrateonly the abstraction, the absence, or non-development of seve-ral intermediary members of the geognostical series. Thegreywacke may, on one hand, dip beneath a felspar rock, orrock of which felspar forms a principal constituent; for ex-ample, beneath transition granite or zircon syenite; and, onthe other hand, be superimposed upon the black limestonecontaining madrepores; but this position shows only the inter-colation of a bed of greywacke between felspar rocks. Sincethe minute investigation of fossil organic bodies has, throughthe important labours of Messrs Cuvier and Brongniart, dif-fused a new life as it were in the study of the tertiary forma-tions, the discovery of the same fossils in analogous beds ofvery distant countries, has rendered the isochronism of verygenerally extended formations still more probable.It is this isochronism alone, this admirable order of succes-sion, which seems given to man to investigate with some degreeof certainty. The attempts which theological geologists havemade to subject the periods to absolute measurements of time,and to connect the chronology of ancient cosmogonic narra-tions with the observations of nature, could not possibly havebeen productive of satisfactory results. “It has been morethan once,” says M. Ramond, in a discourse replete withphilosophical views, “thought that a supplement to our shortannals might be found in the monuments of Nature. Thereis, however, enough of historical ages, to let us see that thesuccession of physical and moral events, is not regulated bythe uniform progress of time, and could not consequently giveit measure. We see behind us a series of creations and de-structions, by means of the strata of which the crust of theearth is composed. They give rise to the idea of so many|107| distinct periods; but these periods, so fertile in events, mayhave been very short, in comparison with the number andimportance of the results. Between the creations and de-structions, on the other hand, we see nothing, however vastthe intervals may be. There, where all is lost in the void ofundetermined antiquity, the degrees of relative age have noappreciable value; because the succession of phenomena hasno longer the scale which relates to the division of time.”(Mémoires de l’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 subdivisionsof gypsums, the strata of marly and oölitic limestone, desig-nated in England by the names of Purbeck Beds, PortlandStone, Coral Rag, Kelloway Rock, and Corn Brash, thenacquire a great degree of importance. Thin beds of second-ary and tertiary formations, containing assemblages of verycharacteristic fossil bodies, have furnished, as it were, a horizonto the geognost. In their prolongation, whatever occurs placedabove or beneath in the order of the whole series, has beenreferred to one of them. Even the particular denominationsby which beds are distinguished, are of much importance in ageognostical description, however whimsical or improper maybe their signification or their origin as taken from the languageof miners. But while treating of the relative position of rockson a surface of great extent, it is indispensably necessary to con-sider the formations, or habitual associations of certain beds,in a more general point of view. It is then that discretion andcircumspection are more necessary in the distinction of rocks,and in their nomenclature. The work of M. Freiesleben on thePlains of Saxony, which are upwards of seven hundred squareleagues in extent (Geogr. Beschr. des Kupferschiefergebirges,1807—15), presents a beautiful model of the union of localobservations and geognostical generalizations. These generali-zations, these attempts to simplify the system of formations,and to direct the attention more especially to great charac-teristic features, should be more or less cautiously conducted,according as one describes the basin of a river, an isolatedprovince, a great country such as France or Germany, or anentire continent.The more minute the investigation of districts becomes, themore does the connexion between formations which appear atfirst perfectly independent, manifest itself by the great phe-nomenon of alternation; that is to say, by a periodical succes-|108| sion of beds which present a certain analogy in their compo-sition, and sometimes even in certain fossil organic bodies. Itis thus that, in the transition-mountains, for example, in America(at the entrance of the plains of Calabozo), beds of greenstoneand euphotide, in Saxony (near Friedrichswalde and Meissen)the clay-slates with glance-coal, the greywackes, porphyries,black limestones, and greenstones, constitute, from their fre-quent and repeated alternation, a single formation. It oftenhappens that subordinate beds appear only at the extremelimits of a formation, and assume the aspect of an independentformation. The cupreous and bituminous marls (Kupferschiefer),which occur in Thuringia between the alpine limestone (zechstein)and the red sandstone (rothes liegende), and which have for agesbeen extensively wrought, are represented in several parts ofMexico, of New Andalusia, and of Southern Bavaria, by mul-tiplied beds of marly clay, more or less carburetted, and in-cluded within the alpine limestone. Similar circumstancesoften give to gypsums, sandstones, and small beds of compactlimestones, the appearance of particular formations. Theirdependence on subordination is known by their frequent asso-ciation with other rocks, by their want of extent and of thick-ness, or by their total suppression, which is frequently observed.It must not be forgot (and this fact has struck me much in thetwo hemispheres) that the great formations of limestones, forexample, the alpine limestone, have their sandstones, as thevery generally extended sandstones have their limestone beds.Thin beds of sandstones, of limestones, and of gypsums, char-acterize, in all the zones, the deposites of coal and rock-saltor muriatiferous clay (salzthon), isolated deposites, which aremost commonly only covered by these small local formations.It is by overlooking these considerations, which should befamiliar to every practical geologist, that the type of the greatindependent formations has been rendered too complicated.The phenomenon of alternation manifests itself, either locallyin rocks, superimposed several times upon each other, andconstituting a single compound formation, or in the series offormations considered in their aggregate. It is either green-stones and syenites, slates and transition limestones, beds oflimestones and of marl, that alternate immediately, or thewhole is a system of mica-slates, and of granular feldspathicrocks (granites, gneisses, and syenites), which reappear amongthe transition deposites, and which separate from the primitivehomonymous system the greywackes and limestones withorthoceratites. For the first knowledge of this fact, one of themost important and least studied of modern geognosy, we are|109| indebted to the beautiful observations of Messrs Leopold vonBuch, Brochant, and Haussmann. From the circumstancethat, in the transition, granular rocks, perfectly destitute oforganic remains, succeed to compact rocks which containthese same remains, it has been concluded by geognosts ofgreat name, that this alternation of shelly and not-shelly rocks,might extend beyond the deposites which we call primitive.It has not been merely asked if the clay-slates, mica-slates,and gneisses, support the granites which have been consideredas the oldest; the question has also been agitated, whethergreywackes and black limestones with madrepores might notrecur beneath those same granites. According to this idea,the primitive and transition rocks would only form a singledeposite; and the first might be regarded as intercalated in adeposite posterior to the development of organic beings, andwhich might penetrate to an unknown depth into the interiorof the globe. I confess, that no direct observation can be asyet adduced in support of these opinions. The fragments ofrocks which I have seen contained in the lithoid lavas of thevolcanoes of Mexico, Quito, and Vesuvius, and which arethought to have been torn from the bowels of the earth, seemto belong to altered rocks of granite, mica-slate, syenite, andgranular limestone, and not to greywackes and limestone withmadrepores.We have preserved in the arrangement of rocks the greatdivisions known by the name of primitive, transition or inter-mediary and tertiary deposites. The natural limits of thesefour systems of rocks are the clay-slate with glance-coal orampelite and lydian stone, alternating with compact limestonesand greywackes, the coal formation, and the formations whichimmediately succeed the chalk. In geognosy, as in descriptivebotany, the subdivisions or small groups of families have moredistinct characters than the great divisions or classes. It isthe case with all the sciences; in which we rise from indi-viduals to species, from species to genera, and from these tostill higher degrees of abstraction. A method necessarilyrests upon differently graduated abstractions, and the passagesbecome more frequent in proportion as the characters are morecomplex. The transition or intermediary formations of Werner,which M. de Buch has first limited with the sagacity for whichhe is distinguished (Moll’s Jahrb. 1798, b. ii. p. 254), are con-nected by the ampelitic clay-slates, the syenites with zircons,the granite sometimes destitute of hornblende, and the anthra-citic mica-slate, with the primitive deposite; while the small-grained greywackes and madreporous and compact limestones,|110| connect them with the coal sandstones and limestones of thesecondary desposites.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 subordinaterocks, and of little thickness. The progressive motion, or, if Imay be allowed to use the expression, the extent of the oscil-lation of the serpentine and euphotide, is very different. Thosediallage rocks, constituting many distinct formations, rarelycovered with other rocks, stop short nearly at the lower boun-dary of the secondary formations; towards the bottom theypenetrate into the primitive deposites to beyond the mica-schist. The chalk seems to present a natural limit to thetertiary formations, which were first characterised by MessrsCuvier and Brongniart, and justly, as deposites entirely dif-ferent from the last secondary formations, described by theFreyberg School (Géogr. Miner. des Environs des Paris, p. 8and 9.) Struck with the relations which exist between thetertiary deposites and the beds beneath the chalk, M. Brong-niart has even recently proposed to designate the tertiary for-mations by the name of upper secondary deposites, (Sur legisement des Ophiolithes, p. 37). Compare also the very inter-esting geognostical discussions contained in M. de Bonnard’sTraité des Roches, p. 138, 210, and 212.)The distinction of four deposites which we have successivelynamed, and of which three are posterior to the developmentof organic life upon the globe, appears to me worthy of beingretained, notwithstanding the passage of some formations toothers of a very different character, and notwithstanding thedoubts which several very distinguished geognosts have foundedupon these passages. The classification of deposites marksgreat epochs of nature; for example, the appearance of somepelagic animals (zoophytes, cephalopodous mollusca), and thesimultaneous destruction of an enormous mass of monocotyle-dons. It presents as it were points of rest to the mind, andby keeping in view that the formations themselves are muchless important than the great divisions, we have often an oppor-tunity, on advancing from high mountains toward the plains,of observing the varied influence which the association ofprimitive and transition rocks, and that of secondary and ter-tiary ones, have exercised upon the inequality and configura-tion of the ground. It is owing to this influence, that the aspectof the landscape, the form of mountains and platforms, and the|111| character of the vegetation, vary less, when we travel parallelto the direction of the beds, than on cutting them at a rightangle. (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 my-self about the impropriety of the greater number of thesedenominations. I continue to separate the clay (with lignites)superimposed upon the chalk, from that which is beneath it,and the chalk itself from the more ancient secondary forma-tions. But these distinctions, by beds and groups of beds, souseful in the description of a deposite of small extent, oughtnot to prevent the geognost, when he tries to rise to a moregeneral point of view, from connecting these clays and thechalk with the Jura limestone, and from regarding them as thelast strata of this great formation, composed of calcareous andmarly beds. The inferior beds of the chalk (tuffeau) containammonities. The limestone of the mountain of St Peter ofMaestricht indicates, as has already been observed by MessrsOmalius and Brongniart (Geogr. Miner., p. 13), the passage ofthe chalk to older secondary limestones. Near Caen, accord-ing to the beautiful observations of M. Prevost, the clays be-neath the chalk contain those same lignites which occur, ingreater quantity, in the clay which is situated immediatelyabove the chalk: cerites, which bring to mind the coarselimestone of Paris, are seen in a limestone with trigonias,placed between clays inferior to the chalk and the ooliticbeds. I do not insist upon these particular facts; I mentionthem only to prove, by a striking example, how, on bringingtogether facts observed in different points of the same country,the great phenomenon of alternation reveals to us the connexionsbetween formations which at first sight appear to have nothingin common. It is the property of those beds which alternatewith one another, of those rocks which succeed each other inperiodical series, to present the most marked contrast in the twobeds which immediately follow each other. In geognosy, asin the different parts of descriptive natural history, the advan-tage of classifications of variously graduated sections must berecognized, without losing sight of the unity of nature; andthose who have contributed the most to the advancement ofnatural philosophy, have possessed at once both the tendencyto generalize, and the exact knowledge of a mass of particu-lar facts.|112| It has been customary to terminate the series of depositesby the volcanic rocks, and to make them succeed the second-ary and tertiary, and even the alluvial deposites. In a sys-tem constructed upon the principle of relative antiquity, thisarragement seems to me to have little to recommend it. It iswithout doubt the case that lithoid lavas are spread over themost recent formations, even over beds of gravel; nor can itbe denied that there exist volcanic productions of differentepochs: but, from what I have observed in the Cordilleras ofPeru, of Quito, and of Mexico, in a part of the world so cele-brated for the frequency of volcanoes, it seems to me, that thechief site of subterranean fires is in the transition rocks, andbeneath those rocks. I have observed, that all the burningor extinct craters of the Andes open in the midst of trapporphyries or trachytes, (Berl. Abhandl. der Kön. Acad. 1813,p. 131), and that these trachytes are connected with the greattransition porphyry, and syenite formation, According to thisobservation, it appears more natural to me to make the second-ary and volcanic deposites to follow the transition deposite ina parallel manner, and as by bisection. By this new arrange-ment, the formation of porphyries, syenites, and greywackes,or that of transition porphyries, syenites, and granites, occursconnected at the same time; 1st, With the porphyries of thered sandstone in the secondary coal-deposite; 2dly, With thetrachytes or trap porphyries which are destitute of quartz,and mixed with pyroxene. I employ with regret the termvolcanic terrain, not that I doubt, like those who designate thetrachytes, basalts and phonolites (porphyrschiefer), by thename of trap terrain, that all which I have associated in thevolcanic terrain has not been produced or altered by fire;but because several rocks, intercalated between the (primi-tive?) transition and secondary rocks, might also be volcanic.I would rather wish to avoid every (historical) idea of theorigin of things, in a (statistical) view of relative situation orsuperposition. At Skeen, in Norway, a basaltic and poroussyenite, containing pyroxenes, is placed, according to the obser-vation of M. de Buch, between the transition limestone andthe syenite with zircons. It is a bed, not a dike; and thisis a much less problematical phenomenon than the basalt(Urgrünstein, Buch. Geogn. Beob. p. 124, and Raumer, Granitdes Riesengebirges, p. 70), contained in the mica-schist ofKrobsdorf in Silesia. The trachytes, with obsidian of Mexico,are intimately connected with the transition porphyries whichalternate with syenites. The amygdaloid belonging to thered sandstone, assumes, on the Continent of Europe, and in|113| Equinoctial America, all the appearance of an amygdaloid ofthe basaltic formation. M. Boué, in his interesting EssaiGeologique sur l’Ecosse, p. 126, 162, has described pyroxenicrocks (dolerites) included in the red sandstone. Withoutprejudging any thing regarding the origin of these masses, orin general regarding that of all the primitive and transitionrocks, we designate here, by the name of Volcanic Terrains,the least 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 beapprehensive of adding new difficulties to those which thediscussion of relative positions already presents. I have, how-ever, carefully avoided the denominations, too long preserved,of under and upper limestone; of gypsum of the first, second, orthird formation, of old and new red sandstone, &c. These de-nominations without doubt present a true geognostical charac-ter; they have relation, not to the composition of rocks, butto their relative age. However, as the general type of theformations of Europe cannot be modelled after that of a singledistrict, the necessity of admitting parallel formations (sichvertretende Gebirgsarten), renders the names of first or secondgypsum, of old or middle sandstone, extremely vague and ob-scure. In one country it is proper to consider a bed of gyp-sum or of common sandstone as a particular formation; whilein another, it should be regarded as subordinate to neighbor-ing formations. The geographical denominations are withoutdoubt the best; they give rise to precise ideas of superposi-tion. When it is said that a formation is identical with theporphyry of Christiania, the lias of Dorsetshire, the sandstoneof Nebra (bunter sandstein), the coarse limestone of Paris,these assertions leave no doubt in the mind of an experiencedgeognost, regarding the position which is to be assigned to theformation in question. It is also by silent convention, as itwere, that the words zechstein of Thuringia, Derbyshire Limestone,Paris Formations, &c. have been introduced into mineralogi-cal language. They express a limestone which immediatelysucceeds the red sandstone of the coal deposite, a transitionlimestone placed beneath the coal sandstone, and lastly, for-mations of more recent origin than the chalk. The only dif-ficulties which the multiplicity of these geographical denomi-nations presents, consist in the choice of names, and in thedegree of certainty which may have been acquired, regardingthe position or relative age of the rock to which the others are|114| referred. The English geognosts look upon the continent fortheir lias and red marl; the German for their bunte sandsteinand muschelkalk. These words present themselves in theminds of travellers associated with remembrances of localities.It is not of so much importance, therefore, to produce preciseideas, as to make choice of localities generally known, andwhich are celebrated, either by the working of mines, or bydescriptive works.In order to diminish the effects of national vanity, and toattach new names to more important objects, I proposed a longtime ago, (1795), the denomination of Alpine Limestone, andJura Limestone. A portion of the High Alps of Switzerland,and the greater part of Jura, are without doubt formed ofthese two rocks: the names, however, generally received atthe present day, of Alpine Limestone (Zechstein), and JuraLimestone, should in my opinion be modified or entirely aban-doned. The lower beds of the Jura mountains, filled withgryphites, belong to an older formation, perhaps to the zech-stein; and a great part of the limestone of the Alps of Swit-zerland assuredly is not zechstein; but, according to Messrsde Buch and Escher, transition limestone. It would thereforebe better to choose the geographical names of rocks fromamong the names of isolated mountains, the whole visible massof which belongs only to a single formation, than to derivethem, as I have erroneously done, from entire chains. I havethought, and many geognosts have formed the same opinion,that the Jura limestone (cavernous limestone of Franconia)was generally placed upon the continent, beneath the Nebrasandstone, (bunte sandstein), between this sandstone and thezechstein. Subsequent observations have proved, that thename of Jura Limestone had with reason been applied torocks which are very distant from the mountains of WesternSwitzerland; but that the true geognostical place of this for-mation, (when there is not a suppression of the inferior forma-tions), occurs above the Nebra sandstone, between the shell-limestone (muschelkalk, or the quadersandstein), and thechalk. A geographical name, justly applied to several analo-gous rocks, renders us attentive to their identity of relativeposition; but the place which homonymous rocks ought tooccupy in the total series, is not well determined except whenthe geographical name has been selected, after having ac-quired a perfect certainty regarding their position. Circum-stances are the same with regard to the relative age of themolasse of Argovia (nagelfluhe) and of the Pirna quadersand-stein (grès blanc of M. de Bonnard), two rocks of very recent|115| origin, which have been very well examined separately, butwhose relations to each other, and to the chalk and Juralimestone, have only been illustrated of late. One may there-fore be pretty sure of having met in the New Continent withrocks identical with the molasse or quadersandstein, withoutbeing able to pronounce with certainty upon their relationswith all the secondary or tertiary rocks. When rocks arenot immediately in contact, and are not covered by depositesof known position, their relative age can only be conjecturedfrom simple analogies.The terms of the geognostical series are either simple orcomplex. To the simple terms belong the greater number ofthe primitive formations: the granites, gneisses, mica-slates,clay-slates, &c. The complex terms occur in greater num-bers among the transition rocks: there, each formation includesan entire group of rocks, which alternate periodically. Theterms of the series are not transition limestones or greywackes,constituting independent formations; they are associations ofclay-slate, green-stone, and greywacke; of porphyry and grey-wacke; of granular and steatitic limestone, and of conglomerates,composed of primitive rocks; of clay-slate and black limestone.When these associations are formed of three or four rockswhich alternate, it is difficult to give them significant names,—names indicative of the whole composition of the group,—ofall the partial members of the complex term of the series. Itmay then assist in fixing the groups in the memory, to retracethe rocks which predominate in them, without being absolutelywanting in the neighbouring groups. It is in this manner that thegranular steatitic limestone characterises the Tarantaise forma-tion; the greywacke, the great transition formation of the Hartzand of the banks of the Rhine; the metalliferous porphyriesrich in hornblende, and almost destitute of quartz, the forma-tion of Mexico and of Hungary. If these phenomena of alter-nation attain their maximum in the transition districts, stillthey are not entirely excluded from the primitive and second-ary terrain. In both of these terrains, complex terms aremixed with the simple terms of the geognostical series. Ishall mention among the secondary formations, the sandstoneplaced below the alpine limestone, (the Nebra sandstone, buntesandstein), which is an association of marly clay, sandstoneand oolites; the limestone which covers the red-sandstone ofthe coal-formation (the zechstein or alpen-kalkstein), which isa less constant association of limestone, of (muriatiferous) gyp-sum, of stinkstone and of pulverulent bituminous marl. In theprimitive class we find the three first terms of the series; the|116| oldest rocks either isolated, or alternating two and two, ac-cording as they are geognostically more approximated bytheir relative age, or the whole three alternating. The granitesometimes forms constant associations with the gneiss, and thegneiss with the mica-slate. These alternations follow particu-lar laws: we see, (for example in Brazil, and, although lessdistinctly, in the maritime chain in Venezuela), the granite,gneiss, and mica-schist in a triple association; but I have notfound 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 analysisapproximates the clay-slate to mica. (Journal de Physique,vol. lxviii. p. 128; Traité de Geognosie, vol. ii. p. 97). Thefirst, it is true, has not the metallic lustre of mica-slate; itcontains a little less potash, and more carbon; the silex doesnot unite into nodules or thin laminæ of quartz, as in the mica-slate; but it cannot be doubted, that scales of mica form theprincipal base of the clay-slate. These scales are so joinedtogether, that the eye cannot distinguish them in the mass.It is perhaps this same affinity which prevents the alternationof clay-slates and mica-slates: for in these alternations Natureseems to favor the association of heterogeneous rocks; or, tomake use of a figurative expression, she delights in the asso-ciations whose alternating rocks present a great contrast ofcrystallization, of mixture, and of colour. At Mexico, I haveseen dark greenstones, alternate thousands of times withreddish-white syenites, abounding more in quartz than in fel-spar. In this greenstone there were veins of syenite, and inthe syenite veins of greenstone; but none of the two rockspassed into the other. (Essai politique sur la Nouvelle Espagne,v. ii. p. 523). They present at the limit of their mutual con-tact, differences as marked as the porphyries which alternatewith the greywackes or with the syenites, as the black lime-stones which alternate with the transition clay-slates, and somany other rocks of entirely heterogeneous composition andaspect. Further, when, in primitive deposites, rocks morerelated by the nature of their composition than by their struc-ture or mode of aggregation, for example, the granites andgneisses, or the gneisses and mica-schists, alternate; these|117| rocks do not by any means show the same tendency to passinto each other, as they present, when isolated in formationswhich are not of a complex character. We have alreadyobserved, that often a bed β, becoming more frequent in therock α, announces to the traveller that the simple formation α,is to be succeeded by a compound formation, in which α andβ alternate. Farther on, it happens, that β assumes a greaterdevelopment; that α is no longer an alternating rock, but asimple bed subordinate to β, and that this rock β shows itselfalone, until, by the frequent repetition of beds γ, it becomesthe precursor of a compound formation β, alternating with γ.We might substitute for these signs the words granite, gneiss,and mica-slate; those of porphyry, greywacke, and syenite;of gypsum, marl, and fetid limestone (stinkstein). Pasigraphiclanguage has the advantage of generalizing the problems; itis more conformable to the wants of geognostical philosophy, ofwhich I attempt to present here the first elements, in so far asthey have relation to the study of the superposition of rocks.Now, if often between formations which are simple and veryclosely allied, in the order of their relative antiquity, betweenthe formations α, β, γ, there occur compound formations inter-posed, αβ and βγ, (that is to say, α alternating with β, and βalternating with γ); we observe, also, although less frequently,that a formation (for example α), assumes so extraordinary anincrease, that it envelopes the formation β; and that β, insteadof showing itself as an independent rock, placed between α andγ, is now nothing but a bed in α. It is thus, that, in LowerSilesia, the red-sandstone contains the formation zechstein; forthe limestone of Runzendorf, filled with impressions of fishes,and analogous to the bituminous marl, abounding in fishes, ofThuringia, is entirely developed in the coal-formation. (Buch,Beob. vol. i. p. 104, 157; Id. Reise nach Norwegen, vol. i. p.158; Raumer, Gebirge von Nieder-schlesien, p. 79). M. Beudant,Voy. Miner., vol. iii. p. 183., has observed a similar phenome-non in Hungary. In other districts, for example, in Switzer-land, at the southern extremity of Saxony, the red-sandstonedisappears entirely; because it is replaced, and, so to speak,overcome by a prodigious development of greywacke or ofalpine limestone. (Freiesleben, Kupfersch. p. 109). Theseeffects of the alternation or unequal development of rock, areso much the more worthy of attention, that their study maythrow light upon some apparent deviations from a generallyacknowledged type of superposition, and that it may serve toa common type the series of position observed in very distantcountries.|118| 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 the morecommonly adopted expressions of granite-gneiss, syenite-green-stone. I was apprehensive that this last method of designatingformations composed of alternating rocks, might rather giverise 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 appre-ciated, M. Nose, has already made use of the words granite-porphyries and porphyry-granites, to indicate varieties of struc-ture and aspect, to separate the porphyritic granites fromporphyries, which, from the frequency of crystals imbeddedin the mass, presents an aggregational, a true granitic structure.By adopting the denominations of granite and gneiss, of syeniteand porphyry, of greywacke and porphyry, of limestone andclay-slate, no doubt is left regarding the nature of the complexterms of the geognostical series.*M. Humboldt next proceeds to consider the natural historyof fossil organic remains, as connected with formations.

* Translated from Essai Geognostique par Baron Alexandre de Humboldt.

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