On Rock Formations.

By Baron Alexander Humboldt .

Translated from Essai Geognostique par Alexandre de Humboldt.

THE word formation designates, in Geognosy, either the manner in which a rock has been produced, or an assemblage of mineral masses, which are so connected together, that they are supposed to have been formed at the same period, and present, in the most distant parts of the world, the same general relations of situation and position.

It is thus that the formation of obsidian and basalt is attributed to subterranean fires; and thus also that we say the formation of 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 the genius of the language; but it has relation to the origin of things, to an uncertain science founded upon geogonic hypothesis.

The second acceptation, now generally adopted by the French mineralogists, has been borrowed from the celebrated school of Werner: it indicates what is, not what is supposed to have been.

In the geognostical description of the globe, we may distinguish 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 associated, and which present the same relations of position, constitute a formation; the union of several formations constitutes a district or terrain; but these different terms of rocks, Formation and Terrain, are employed as synonymous in many works of geognosy.

The diversity of the rocks, and the relative disposition of the 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 deposit of salt, of coal, or of clay-iron, which was covered with a great number of beds of different natures, it gave rise to ideas more or less precise regarding the system of rocks peculiar to a district of small extent.

Furnished with these local details, and full of prejudices which arise from custom, the miners of a country would disperse themselves over the neighbouring districts.

They would do what geognosts have often done in our days; they would judge of the position of rocks of whose nature they were ignorant, according to imperfect analogies, according to the circumscribed ideas which they had acquired in their native country.

This error must have had a fatal influence upon the success of their new researches.

In place of examining the connection of two contiguous districts, by following some generally extended bed,--in place of enlarging and extending, so to speak, the first type of formations which had remained impressed upon their minds,--they would be persuaded that each portion of the globe had an entirely different geological constitution.

This very old popular opinion has been adopted and supported, 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, and journeys made into distant countries, have presented a more exact comparison of different districts, great and immutable laws have been discovered in the structure of the globe, and in the superposition of rocks.

Since, then, the most striking analogies of situation, of composition, and of organic bodies contained in contemporaneous beds, have manifested themselves in the two worlds, in proportion as we become accustomed to consider the formations under a more general point of view, even their identity becomes every day more probable.

In fact, on examining the solid mass of our planet, we perceive that some of those substances with which oryctognosy (descriptive mineralogy), makes us acquainted in their individual capacities, are met with in constant associations, and that these associations, which are designated by the name of Compound Rocks, do not vary, like organic beings, according to the differences of the latitudes, or of the isothermal lines in which they occur.

The geognosts who have travelled over the most remote countries, have not only met in the two hemispheres with the same simple substances, quartz, felspar, mica, garnet or hornblende; but they have also found that the great mountain masses present almost everywhere the same rocks, that is to say, the same assemblages of mica, quartz and felspar in the granite; of mica, quartz, and garnets in the mica-slate; of felspar and hornblende in the syenite. If it has sometimes been thought at first that a rock belonged exclusively to a single portion of the globe, it has been constantly found by later researches, in regions the most remote from its first locality.

We are tempted to admit that the formation of rocks has been independent of the diversity of climates; that perhaps it has even been anterior to them, (Humboldt, Geographie des Plantes, 1807, p. 115.; Vues des Cordilleres, vol. i. p. 122).

Rocks are found to be identical where organic beings have undergone the most varied modifications.

But this identity of composition, this analogy which is observed in the association of certain simple mineral substances, might be independent of the analogy of relative situation and of superposition.

One may have brought from the Islands of the Pacific Ocean, or from the Cordilleras of the Andes, the same rocks which are observed in Europe, without his being permitted to conclude that these rocks are superimposed in the same manner, and that after the discovery of one of them it might be predicted with some degree of certainty what are the other rocks which occur in the same places.

It is to discover these analogics of situation and relative position, that the labours of geognosts should tend, who delight to investigate 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 formations will not only be extended, but also variously modified, in proportion as the number of travellers qualified to make geognostical observations shall become increased, and as complete monographs of different districts at great distances from each other shall furnish more precise results.

The exposition of the laws observed in the superposition of rocks, forms the most solid part of the science of geognosy.

It must not be denied, that the observations of geognostical situation often present great difficulties, when the point of contact of two neighbouring formations cannot be reached, or when they do not present a regular stratification, or when their relative situation is not uniform, that is to say, when the strata of the upper deposits are not parallel to the strata of the lower.

But these difficulties (and this is one of the great advantages of observations which embrace a considerable part of our planet), diminish in number, or disappear entirely, on comparing several districts of great extent.

The superposition and relative age of rocks, are facts susceptible of being established immediately, like the structure of the organs of a vegetable, like the proportions of elements in chemical analysis, or like the elevation of a mountain above the level of the sea.

True geognosy makes known the outer crust of the globe, such as it exists at the present day.

It is a science as capable of certainty as any of the physical descriptive sciences can be.

On the other hand, all that relates to the ancient state of our planets, to those fluids which, it is said, held all the mineral substances in a state of revolution, to those seas which we have raised to the summit of the Cordilleras, to make them again disappear, is as uncertain as are the formation of the atmosphere of planets, the migrations of vegetables, and the origin of different varieties of our species.

Yet the period is not very remote when geologists occupied themselves by preference with the solution of these almost impossible problems, with those fabulous times of the physical history of our planet.

In order to render the principles better understood, according to which the following table of the superposition of rocks is constructed, it becomes necessary to premise observations furnished by the practical examination of different districts.

We shall begin with remarking, that it is not easy to circumscribe the limits of a formation.

The Jura limestone and the Alpine limestone, which are separated to a great distance in one country, sometimes appear closely connected in another.

What announces the independence of a formation, as has been very justly observed by M. de Buch, is its immediate superposition upon rocks of a different nature, and which consequently ought to be considered as more ancient.

The red sandstone is an independent formation, because it is superimposed indifferently upon black (transition) limestone, upon mica-slate, or upon primitive granites; but in a country where the great formation of syenite and porphyry predominates, these two rocks constantly alternate.

There results that the syenite rock is dependent upon the porphyry, and scarcely any where covers by itself the transition clay-slate or primitive gneiss.

The independence of formations does not, besides, by any means exclude the uniformity or concordance of position; it rather excludes the oryctognostic passage of two superimposed formations.

The transition districts have very often the same direction and the same inclination as the primitive ones; and yet, whatever approximation there may be between their origin, we are not the less warranted to consider the anthracitic mica-slate or the grey-wacke, alternating with porphyry, as two formations independent of the primitive granites and gneisses which they cover.

The conformity of position is in no way incompatible with the independence of formations, that is to say, it does not prevent the right which one has of regarding a rock as a distinct formation.

It is because the independent formations are placed indifferently on all the older rocks, (the chalk upon the granite, the red sandstone upon the primitive mica-slate), that the assemblage of a great number of observations made upon very distant points, becomes eminently useful in the determination of the relative age of rocks.

In order to determine that the zircon-sienite is a transition rock, it must have been seen resting upon formations posterior to the black limestone with orthoceratites.

Observations made upon the porphyries and syenites of Hungary by M. Beudant, one of the most distinguished geologists of the present times, may throw much light upon the formations of the Mexican Andes. It is thus that a new vegetable discovered in India, elucidated the natural affinity between two families of plants belonging 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 not pretend that this position is observed in all the countries of the globe; I merely point it out such as it has appeared the most probable, after the comparison of a great number of facts which I have collected.

It is by the idea of the relative age of formation, that I have been guided in this work, imperfect as it still is. I had begun it long before my journey to the Cordilleras of the New Continent, from the year 1792, when, on leaving the Freyberg School, I was appointed to the direction of the Mines in the mountains of the Fichtelgebirge.

The same rock may vary in composition, integrant parts may have been abstracted, and new substances may occur disseminated, without the rock's changing its denomination in the eyes of the geognost who is engaged with the superposition of formations.

Under the equator, as in the north of Europe, strata of a true transition syenite lose their hornblende, without the mass becoming another rock.

The granites of the banks of the Orinoco sometimes assume hornblende as an integrant part, and yet do not cease to be primitive granite, although this may not be of the first or oldest formation.

These facts have been observed by all practical geologists.

The essential character of the 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 Begriff einer, in the Mag. der Naturf., 1810, p. 128-133.) It is on this account that an isolated fragment, a specimen of rock found in a collection, cannot be determined geognostically, that is to say, it cannot be referred with certainty to a particular formation, constituting one of the numerous beds of which the crust of our planet is composed.

The presence of chiastolite, the accumulation of carbon or nodules of compact limestone in the clay-slates, nigrine and epidote in the syenites, (alternating with granite and porphyries), conglomerates or pudding-stones, having a basis of anthracitic mica-slate, may, without doubt, be characteristic of transition formations; in the same way as, according to the useful labours of M. Brongniart, petrifactions of shells, in a good state of preservation, sometimes indicate directly such or such a bed of tertiary deposits.

But these cases, where we are guided by disseminated 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, and from small veins of carbonate of lime, which traverse calcareous rocks; those which are furnished by the fissility and silky lustre of clay-slate, the aspect and undulations more or less marked of the scales of mica in mica-slate; and, lastly, the size and colouring of the crystals of felspar in the granites of different formations, may, like all that is connected simply with the physiognomy of minerals, lead the most expert observer into error.

The white and black tints undoubtedly in most instances distinguish the primitive and transition limestones; the Jura formation, especially in its upper beds, is also without doubt generally divided into thin whitish beds, having a dull, even, or conchoidal fracture, with very flat cavities:

but in the mountains of transition limestone there are isolated masses which, in colour and texture, resemble the oryctognostic characters of the Jura limestone; and to the south of the Alps there are hills belonging to tertiary deposits, where we find rocks analogous to the slaty and dull Jura limestone (in as far as regards appearance,) in formations placed above the chalk, and which resemble the limestone used for lithographic purposes.

Were names taken from their oryctognostical characters alone to be preferred in distinguishing formations, the different strata of the same compound rock having a considerable thickness, and extended to a great length in a particular direction, would often seem to belong to different rocks, according to the points at which specimens were taken.

Consequently we can only determine geognostically in collections, suites of rocks of which the mutual superposition is known.

In announcing these ideas regarding the sense which should be attached to the words independent formations, when treating of the order of their position, we are very far from undervaluing the eminent services which the most rigorous oryctognostic examination, the minute investigation of the composition of rocks, have rendered to modern geognosy, and especially to the knowledge of the relative position of formations.

Although, according to the discoveries of M. Haüy, regarding the intimate nature of inorganic and crystallized substances, there does not exist, properly speaking, a passage or transition of one mineral species to another; (Cordier, sur les Roches volcan., p. 33., and Berzelius, Nouv. Syst. de Mineral, p. 119.), the passages of masses or pastes of rocks, are not limited to formations which are commonly distinguished by the name of Compound Rocks.

Those which are thought simple, for example, the transition or secondary limestones, are partly amorphous varieties of mineral species, of which there exists a crystallized type, partly of aggregates of clay, carbon, &c., which cannot be submitted to any fixed determination.

It is upon the variable proportions of these heterogeneous mixtures, that the passage of marly limestones to other schistose formations is founded.

(Haüy, Tableau comparatif de la Cristallographie, p. 27.--30.)

All the amorphous pastes of rocks, however homogeneous they appear at first sight, the bases of porphyries and euphotides (serpentines), as well as those problematical black masses which constitute the basanite (basalt) of the ancients, and which are not all greenstones surcharged with hornblende, are susceptible of being subjected to mechanical analysis. M. Cordier has applied this analysis in an ingenious manner to the diabases, dolerites, and other more recent volcanic productions.

The most apparently minute oryctognostic examination, cannot be indifferent to the geognost who examines the age of formations.

It is by this examination that we are enabled to form a just idea of the progressive manner in which, by internal development, that is to say, by a very gradual change in the proportions of the elements of the mass, the passage is made from one rock to a neighbouring.

The transition slates, whose structure appears at first so different from that of the granites or porphyries, present to the attentive observer striking examples of insensible passages to granular rocks of porphyritic or granitic nature.

These slates become at first greenish and harder.

In proportion as the amorphous paste receives hornblende, it passes into those hornblendic traps which in former times were confounded with basalt.

In other cases, the mica, which is at first concealed in the amorphous paste, becomes developed, and separates into distinct and clearly crystallized spangles; at the same time, the felspar and quartz become visible; and the mass assumes a granular aspect, with very elongated grains: this is a true transition gneiss.

By degrees, the grains lose their common direction; the crystals arrange themselves around many centres; the rock becomes a transition granite or syenite.

In other cases, the quartz alone is developed; 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 of nature have become familiar by long examination, become aware beforehand of the proximity of granular, granitic and arenaceous rocks.

Analogous passages of primitive mica-slate to a porphyritic rock, and the return of this rock to gneiss, are observed in the eastern parts of Switzerland. (See the luminous developments given by M. de Raumer, Fragmente, p. 10. and 47.; M. Leopold de Buch, in his Voyage de Glaris a Chiavenna, fait en 1803, and inserted in the Magaz. der Berl. Naturf., vol. iii. p. 115).

But these passages are not always insensible and progressive; the rocks often also succeed 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, are as distinct as the limits between the porphyries and limestones; but even in this case, geognostical relations with the superimposed rocks are indicated by additional heterogeneous beds.

It is thus that the transition granite of the syenitic formation presents beds of basanite, by becoming charged with hornblende: it is thus, also, that these same granites sometimes pass to euphotide.

(Buch, Voyages en Norwege, vol. i. p. 138., vol. ii. p. 83.)

There results from these considerations, that the mechanical analysis of amorphous pastes, by means of demi-triturations and washings, (an analysis of which, M. Fleuriau de Bellevue made the first attempt, that was crowned with success; Journ.

de Physique, vol. li. p. 162.), throws light at once, 1st, Upon the large crystals which are isolated and separated from the microscopic crystals entangled in the mass; 2dly, On the mutual passages of some rocks, superimposed the one upon the other; 3dly, On the subordinate beds, which are of the same nature as one of the elements of the amorphous mass.

All these phenomena are produced, if we may so speak, by internal development; by variation in the constituent parts of a heterogeneous mass.

Crystalline molecules, invisible to the eye, occur enlarged and disengaged from the compact tissue of the paste; by their assemblage and mixture with new substances, they insensibly become intercalated beds of considerable thickness; and not unfrequently they even become new rocks.

It is the intercalated beds which especially merit the greatest attention.

(Leonhard, Kopp and Gaertner, Propaed. der Miner., p. 158.)

When two formations succeed each other immediately, it happens that the beds of the one begin at first to alternate with the beds of the other, until (after these precursors of a great change) the newest formation shows itself without 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 relative position of the mineral masses.

Their effects belong to the province of geology; but, in order to discover and appreciate them, the observer must call to his assistance oryctognosy.

In exposing the intimate relations by which we often see the phenomena of composition connected with those of relative situation, it has not been my intention to speak of the purely oryctognostic method, which considers rocks according to the analogy of their composition alone.

(Journal des Mines, vol. xxxiv. No. 199.)

In the classifications of this method, abstraction is made of every idea of superposition; but they do not the less give rise to interesting observations regarding the constant assemblage 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 the changes which the mixed rocks undergo, a stratum of great extent and thickness may contain (we must repeat it here) parts to which the oryctognost, who classes rocks according to their composition, would give entirely different denominations.

These remarks have not escaped the learned author of the Classification Mineralogique des Roches; they must have presented themselves to an experienced geognost, who has so successfully investigated the superposition of the deposites of which he has treated.

"We must not confound," says M. Brongniart, in his late Memoir on the position of the Ophiolites, "the relative positions, the orders of superposition of the deposites and of the rocks which compose them, with purely mineralogical descriptions.

The neglect of making the proper distinction in this case, would necessarily be productive of confusion in the science, and would retard its progress."

The arrangement which we give at the end of this article, is by no means what is called a classification of rocks; there will not even be found united, under the title of particular sections (as in the old geognostical method of Werner, or in the excellent Traite de Geognosie of M. D'Aubuisson), all the primitive formations of granite, nor all the secondary formations of sandstone and limestone.

It has been attempted, on the contrary, to place each rock as it occurs in nature, according to the order of its superposition or of its respective age.

The different formations of granite are separted by gneisses, mica-slates, black-limestones and greywackes.

In the transition rocks, we have separated the formations of porphyries and syenites of Mexico and Peru, which are anterior to the grey-wacke, and to the limestone with orthoceratites, from the much more recent formation of the zircon-porphyries and syenites of Scandinavia.

In the secondary rocks, we have separated the oolitic sandstone of Nebra, which is posterior to the alpine limestone or zechstein, from the red-sandstone, which belongs to the same formation with the secondary porphyry and amygdaloid.

According to the principle which we follow, the same names of rocks occur several times in the same table.

Anthracitic mica-slate is separated, by a great number of older formations, from the mica-slate anterior to the primitive clayslate.

Instead of a classification of granitic, schistose, calcareous and arenaceous rocks, it has been my object to present a sketch of the geognostical structure of the globe; a table in which the superimposed rocks succeed each other, from below upwards, as in those ideal sections which I designed in 1804, for the benefit of the Mexican School of Mines, and of which many copies have been distributed since my return to Europe.

(Bosquejo de una Pasigrafia geognostica, con tablas que ensenan la estratificacion y el parallelismo de las rocas en ambos Continentes, para el uso del Real Seminario de Mineria de Mexico.)

These Pasigraphic tables united to my own observations made in both Americas what had at that period been known with precision regarding the relative position of the primitive, intermediary and secondary rocks in the Old Continent.

They presented, together with the type which might be considered as the most general, the secondary types, that is to say, the beds which I have named parallel.

This same method has been followed in the work which I now publish.

My parallel formations are geognostical equivalents; they are rocks which represent each other. (See the Traite de Geologie de M. d'Aubuisson, vol. ii. p. 255.)

In England, and on the opposite Continent, there does not exist an identity of all the formations:

there exist equivalents or parallel formations.

That of our coal situated between the transition masses and the red-sandstone, the position of the rock-salt which occurs on the Continent in the alpine limestone, and the position of our oolites in the Nebra sandstone and Jura limestone, may guide the geognost in the approximation of remote formations.

In England, we observe the coals placed upon transition formations; for example, upon the mountain-limestone of Derbyshire and of South Wales, and upon the transition sandstone, or old redsandstone of Herefordshire. I have thought that I recognised in the magnesian-limestone the red marl, the lias and white oolites of Bath, the united formations of the alpine limestone, of the oolitic sandstone and Jura limestone.

In comparing the formations of countries more or less distant from each other, those of England and of France, for instance, of Mexico and Hungary, of the secondary basin of Santa Fe de Bogota and of Thuringia, we must not think of opposing to each individual rock a parallel one; it must be recollected, that a single formation may represent several others.

It is according to this principle that beds of clay, lying beneath the chalk, may, in France, be separated in the most distinct manner from the oolitic limestone beds; while in Switzerland, in Germany, and in South America, they have for equivalents beds of marls subordinate to the Jura limestone.

The gypsums, which, in one district, are sometimes only intercalated beds in the alpine limestone or oolitic sandstone, in another district, assume all the appearance of independent formations, and occur interposed between the alpine limestone and the oolitic sandstone, between this sandstone and the muschelkalk.

The learned Oxford Professor, Mr Buckland, whose extensive researches have been equally useful to the geognosts of England and of the Continent, has lately published a table of parallel formations, or, as he calls them, equivalents of rocks, which only extends from the 44th to the 54th degree of north latitude, but which merits the greatest attention.

(On the Structure of the Alps, and their relation with the rocks of England, 1821.)

As in the history of ancient nations, it is easier to verify the series of events in each country, than to determine their mutual coincidence; so also more accuracy can be attained in estimating the superposition of formations in isolated regions, than in determining the relative age or parallelism of formations which belong to different systems of rocks.

Even in countries which are not widely separated, in France, in Switzerland, and in Germany, it is not easy to fix the relative antiquity of the muschelkalk, of the molasse of Argovie, and of the quadersandstein of the Hartz; because rocks of general occurrence are here most commonly wanting, which, according to the happy expression of of M. de Grüner, serve as a geognostical horizon, and with which we might compare the three formations in question.

When rocks are not in immediate contact, we can only judge of their parallelism by the relations of age existing between them and other formations by which they are united.

These inquiries of comparative geognosy, will long occupy the sagacity of observers; and it is not surprising that those who set out with the idea of retracing each formation in all the individuality of its relative position, interior structure and subordinate beds, should finish with utterly denying all analogy of superposition.

I had the advantage of visiting, previous to my journey to the Equator, a great part of Germany, of France, of Switzerland, of England, of Italy, of Poland, and of Spain.

During these excursions, my attention was particularly directed to the relative position of formations, a phenomenon which I calculated upon discussing in a special work.

On my arrival in South America, and while at first traversing in different directions the vast deposites which stretch from the maritime chain of Venezuela to the basin of the Amazon, I was singularly struck with the conformity of position which the two Continents present. (See my first sketch of a Geological Table of Equinoctial America, in the Journal 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 latitude to the 12th degree of south latitude, have confirmed these first perceptions.

But in speaking of analogies which are observed in the relative position of rocks, and of the uniformity of those laws which reveal to us the order of Nature, I might adduce a testimony otherwise of more weight than mine, that of the great geognost whose works have thrown the greatest light upon the structure of our globe. M. Leopold de Buch has pushed his researches from the Archipelago of the Canary Isles to beyond the Polar Circle to the 71st degree of latitude.

He has discovered new formations situated between others already known; and, in the primitive as in the transition deposites, in the secondary as well as in the volcanic, he has been struck with the great features by which the table of formations is characterized in the most distant regions.

(To be continued.)

On Rock Formations.

By Baron Alexander Humboldt.

(Concluded from p. 53.)

From that scepticism which would deny the existence of any kind of regular order in the position of rocks, it is proper to distinguish an opinion which has sometimes found supporters among experienced observers.

According to this opinion, the formations of gneiss-granite, of greywacke, of alpine limestone, and of chalk, which have a uniform superposition in different countries, do not very well correspond among themselves as to the age of the homonymous elements of each series.

It is thought that a secondary rock may have been formed on one spot of the globe, while transition rocks did not yet exist on another spot.

In this supposition, no allusion is had to those granitic rocks which are found lying above limestone containing orthoceratites, and which are consequently newer than the primitive rocks.

It is a fact generally admitted at this day, that formations of analogous composition have been repeatedly deposited at epochs far removed from each other.

The doubt which we are now considering, (though we do not partake in it), bears on a point much less clearly established,--the ascertaining whether certain micaslate rocks, evidently situate in the midst of a country of primitive rocks, and placed below those in which the vestiges of organic life begin to appear, are newer than the secondary rocks of another country. I confess, that, in the part of the globe which I have had an opportunity of examining, I have not seen any thing that might tend to confirm this opinion.

Granular syenitic rocks repeated twice, perhaps even three times, in primitive, intermediary (and secondary) deposits, are analogous phenomena with which we have become acquainted within these fifteen years.

The disagreement in regard to age of great homonymous formations, does not by any means seem to me to be proved as yet by direct observations, made upon the contact of superimposed formations.

The chalk or Jura limestone may, on one hand, immediately cover primitive granite, and, on the other, be separated from it by numerous secondary and transition rocks: these very common facts demonstrate only the abstraction, the absence, or non-development of several intermediary members of the geognostical series.

The greywacke may, on one hand, dip beneath a felspar rock, or rock of which felspar forms a principal constituent; for example, beneath transition granite or zircon syenite; and, on the other hand, be superimposed upon the black limestone containing madrepores; but this position shows only the intercalation of a bed of greywacke between felspar rocks.

Since the minute investigation of fossil organic bodies has, through the important labours of Messrs Cuvier and Brongniart, diffused a new life as it were in the study of the tertiary formations, the discovery of the same fossils in analogous beds of very distant countries, has rendered the isochronism of very 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 of certainty.

The attempts which theological geologists have made to subject the periods to absolute measurements of time, and to connect the chronology of ancient cosmogonic narrations with the observations of nature, could not possibly have been productive of satisfactory results.

"It has been more than once," says M. Ramond, in a discourse replete with philosophical views, "thought that a supplement to our short annals might be found in the monuments of Nature.

There is, however, enough of historical ages, to let us see that the succession of physical and moral events, is not regulated by the uniform progress of time, and could not consequently give it measure.

We see behind us a series of creations and destructions, by means of the strata of which the crust of the earth is composed.

They give rise to the idea of so many distinct periods; but these periods, so fertile in events, may have been very short, in comparison with the number and importance of the results.

Between the creations and destructions, on the other hand, we see nothing, however vast the intervals may be.

There, where all is lost in the void of undetermined antiquity, the degrees of relative age have no appreciable value; because the succession of phenomena has no longer the scale which relates to the division of time." (Memoires 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 with sufficient minuteness the different beds which compose the local formations, shelving banks of sand and clay, the subdivisions of gypsums, the strata of marly and oölitic limestone, designated in England by the names of Purbeck Beds, Portland Stone, Coral Rag, Kelloway Rock, and Corn Brash, then acquire a great degree of importance.

Thin beds of secondary and tertiary formations, containing assemblages of very characteristic fossil bodies, have furnished, as it were, a horizon to the geognost.

In their prolongation, whatever occurs placed above or beneath in the order of the whole series, has been referred to one of them.

Even the particular denominations by which beds are distinguished, are of much importance in a geognostical description, however whimsical or improper may be their signification or their origin as taken from the language of miners.

But while treating of the relative position of rocks on a surface of great extent, it is indispensably necessary to consider the formations or habitual associations of certain beds in a more general point of view.

It is then that discretion and circumspection are more necessary in the distinction of rocks, and in their nomenclature.

The work of M.

Freiesleben on the Plains of Saxony, which are upwards of 700 square leagues in extent (Geogr. Beschr. des Kupferschiefergebirges, 1807--15), presents a beautiful model of the union of local observations and geognostical generalizations.

These generalizations, these attempts to simplify the system of formations, and to direct the attention more especially to great characteristic features, should be more or less cautiously conducted, according as one describes the basin of a river, an isolated province, a great country such as France or Germany, or an entire continent.

The more minute the investigation of districts becomes, the more does the connection between formations which appear at first perfectly independent, manifest itself by the great phenomenon of alternation; that is to say, by a periodical succession of beds which present a certain analogy in their composition, and sometimes even in certain fossil organic bodies.

It is thus that, in the transition-mountains, for example, in America (at the entrance of the plains of Calabozo), beds of greenstone and euphotide, in Saxony (near Friedrichswalde and Meissen) the clay-slates with glance-coal, the greywackes, porphyries, black limestones, and greenstones, constitute, from their frequent and repeated alternation, a single formation.

It often happens that subordinate beds appear only at the extreme limits of a formation, and assume the aspect of an independent formation.

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 ages been extensively wrought, are represented in several parts of Mexico, of New Andalusia, and of Southern Bavaria, by multiplied beds of marly clay, more or less carburetted, and included within the alpine limestone.

Similar circumstances often give to gypsums, sandstones, and small beds of compact limestones, the appearance of particular formations.

Their dependence on subordination is known by their frequent association with other rocks, by their want of extent and of thickness, or by their total suppression, which is frequently observed.

It must not be forgot (and this fact has struck me much in the two hemispheres) that the great formations of limestones, for example, the alpine limestone, have their sandstones, as the very generally extended sandstones 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 by these small local formations.

It is by overlooking these considerations, which should be familiar to every practical geologist, that the type of the great independent formations has been rendered too complicated.

The phenomenon of alternation manifests itself, either locally in rocks, superimposed several times upon each other, and constituting a single compound formation, or in the series of formations considered in their aggregate.

It is either greenstones and syenites, slates and transition limestones, beds of limestones and of marl, that alternate immediately, or the whole is a system of mica-slates, and of granular feldspathic rocks (granites, gneisses, and syenites), which reappear among the transition deposits, and which separate from the primitive homonymous system the greywackes and limestones with orthoceratites.

For the first knowledge of this fact, one of the most important and least studied of modern geognosy, we are indebted to the beautiful observations of Messrs Leopold von Buch, Brochant, and Haussmann.

From the circumstance that, in the transition system, granular rocks, perfectly destitute of organic remains, succeed to compact rocks which contain these same remains, it has been concluded by geognosts of great 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 considered as the oldest; the question has also been agitated, whether greywackes and black limestones with madrepores might not recur beneath those same granites.

According to this idea, the primitive and transition rocks would only form a single deposite; and the first might be regarded as intercalated in a deposite posterior to the development of organic beings, and which might penetrate to an unknown depth into the interior of the globe. I confess, that no direct observation can be as yet adduced in support of these opinions.

The fragments of rocks which I have seen contained in the lithoid lavas of the volcanoes of Mexico, Quito, and Vesuvius, and which are thought to have been torn from the bowels of the earth, seem to belong to altered rocks of granite, mica-slate, syenite, and granular limestone, and not to greywackes and limestone with madrepores.

We have preserved in the arrangement of rocks the great divisions known by the name of primitive, transition or intermediary, secondary and tertiary deposites.

The natural limits of these four systems of rocks are the clay-slate with glancecoal or ampelite and lydian stone, alternating with compact limestones and greywackes, the coal formation, and the formations which immediately succeed the chalk.

In geognosy, as in descriptive botany, the subdivisions or small groups of families have more distinct characters than the great divisions or classes.

It is the case with all the sciences; in which we rise from individuals to species, from species to genera, and from these to still higher degrees of abstraction.

A method necessarily rests upon differently graduated abstractions, and the passages become more frequent in proportion as the characters are more complex.

The transition or intermediary formations of Werner, which M. de Buch has first limited with the sagacity for which he is distinguished (Moll's Jahrb. 1798, b. ii. p. 254.), are connected by the ampelitic clay-slates, the syenites with zircons, the granite sometimes destitute of hornblende, and the anthracitic mica-slate, with the primitive deposite; while the smallgrained greywackes and madreporous and compact limestones, connect them with the coal sandstones and limestones of the secondary deposites.

Porphyries of very different formations have their principal seat among the transition rocks; but they break out, if we may so speak, in considerable masses towards the secondary deposites, where they are connected with the coal sandstone, while they penetrate into the primitive class only as subordinate rocks, and of little thickness.

The progressive motion, or, if I may be allowed to use the expression, the extent of the oscillation of the serpentine and euphotide, is very different.

Those diallage rocks, constituting many distinct formations, rarely covered with other rocks, stop short nearly at the lower boundary of the secondary formations; towards the bottom they penetrate into the primitive deposites to beyond the mica-schist.

The chalk 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 secondary formations, described by the Freyberg School (Geogr. Miner. des Environs des Paris, p. 8. and 9.) Struck with the relations which exist between the tertiary deposites and the beds beneath the chalk, M. Brongniart has even recently proposed to designate the tertiary formations by the name of upper secondary deposites, (Sur le gisement des Ophiolithes, p. 37).

Compare also the very interesting geognostical discussions contained in M. de Bonnard's Traite des Roches, p. 138, 210, and 212.)

The distinction of four deposites which we have successively named, and of which three are posterior to the development of organic life upon the globe, appears to me worthy of being retained, notwithstanding the passage of some formations to others of a very different character, and notwithstanding the doubts which several very distinguished geognosts have founded upon these passages.

The classification of deposites marks great epochs of nature; for example, the first appearance of some pelagic animals (zoophytes, cephalopodous mollusca), and the simultaneous destruction of an enormous mass of monocotyledons.

It presents as it were points of rest to the mind, and by keeping in view that the formations themselves are much less important than the great divisions, we have often an opportunity, on advancing from high mountains toward the plains, of observing the varied influence which the association of primitive and transition rocks, and that of secondary and tertiary ones, have exercised upon the inequality and configuration of the ground.

It is owing to this influence, that the aspect of the landscape, the form of mountains and platforms, and the character of the vegetation, vary less, when we travel parallel to the direction 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, Brochant, Beudant, Buckland, Raumer (Geb. von Nieder-Schles., 1819), and other celebrated geognosts, to group the independent formations according to the divisions, into primitive, transition, secondary, and tertiary deposites, without troubling myself about the impropriety of the greater number of these denominations.

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 formations.

But these distinctions, by beds and groups of beds, so useful in the description of a deposite of small extent, ought not to prevent the geognost, when he tries to rise to a more general point of view, from connecting these clays and the chalk with the Jura limestone, and from regarding them as the last strata of this great 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 indicates, as has already been observed by Messrs Omalius and Brongniart (Geogr. Miner., p. 13), the passage of the chalk to older secondary limestones.

Near Caen, according to the beautiful observations of M. Prevost, the clays beneath the chalk contain those same lignites which occur, in greater quantity, in the clay which is situated immediately above the chalk: cerites, which bring to mind the coarse limestone of Paris, are seen in a limestone with trigonias, placed between clays inferior to the chalk and the oolitic beds.

I do not insist upon these particular facts; I mention them only to prove, by a striking example, how, on bringing together facts observed in different points of the same country, the great phenomenon of alternation reveals to us the connections between formations which at first sight appear to have nothing in common.

It is the property of those beds which alternate with one another, of those rocks which succeed each other in periodical series, to present the most marked contrast in the two beds which immediately follow each other.

In geognosy, as in the different parts of descriptive natural history, the advantage of classifications of variously graduated sections must be recognised, without losing sight of the unity of nature; and those who have contributed the most to the advancement of natural philosophy, have possessed at once both the tendency to generalize, and the exact knowledge of a mass of particular facts.

It has been customary to terminate the series of deposites by the volcanic rocks, and to make them succeed the secondary and tertiary, and even the alluvial deposites.

In a system constructed upon the principle of relative antiquity, this arrangement seems to me to have little to recommend it. It is without doubt the case that lithoid lavas are spread over the most recent formations, even over beds of gravel; nor can it be denied that there exist volcanic productions of different epochs: but, from what I have observed in the Cordilleras of Peru, of Quito, and of Mexico, in a part of the world so celebrated for the frequency of volcanoes, it seems to me, that the chief site of subterranean fires is in the transition rocks, and beneath those rocks.

I have observed, that all the burning or extinct craters of the Andes open in the midst of trap porphyries or trachytes, (Berl. Abhandl. der Kön. Acad., 1813, p. 131), and that these trachytes are connected with the great transition porphyry, and syenite formation.

According to this observation, it appears more natural to me to make the secondary and volcanic deposites to follow the transition deposite in a parallel manner, and as by bisection.

By this new arrangement, the formation of porphyries, syenites and greywackes, or that of transition porphyries, syenites and granites, occurs connected at the same time; 1st, With the porphyries of the red sandstone in the secondary coal-deposite; 2dly, With the trachytes or trap porphyries which are destitute of quartz, and mixed with pyroxene.

I employ with regret the term volcanic terrain, not that I doubt, like those who designate the trachytes, basalts and phonolites (porphyrschiefer), by the name of trap terrain, that all which I have associated in the volcanic terrain has not been produced or altered by fire; but because several rocks, intercalated between the (primitive?) transition and secondary rocks, might also be volcanic.

I would rather wish to avoid every (historical) idea of the origin of things, in a (statistical) 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, between the transition limestone and the syenite with zircons.

It is a bed, not a dike; and this is a much less problematical phenomenon than the basalt (Urgrünstein, Buch.

Geogn. Beob. p. 124, and Raumer, Granit des Riesengebirges, p. 70.) contained in the mica-schist of Krobsdorf in Silesia.

The trachytes, with obsidian of Mexico, are intimately connected with the transition porphyries which alternate with syenites.

The amygdaloid belonging to the red sandstone, assumes, on the Continent of Europe, and in Equinoctial America, all the appearance of an amygdaloid of the basaltic formation. M. Boue, in his interesting Essai Geologique sur l'Ecosse, p. 126, 162, has described pyroxenic rocks (dolerites) included in the red sandstone.

Without prejudging any thing regarding the origin of these masses, or in general regarding that of all the primitive and transition rocks, 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 have made use of the names most generally employed by the geognosts of France, Germany, England, and Italy: in attempting to perfect the nomenclature of formations, I should be apprehensive of adding new difficulties to those which the discussion of relative positions already present.

I have, however, carefully avoided the denominations, too long preserved, of under and upper limestone; of gypsum of the first, second, or third formation, of old and new red sandstone, &c. These denominations without doubt present a true geognostical character; they have relation, not to the composition of rocks, but to their relative age.

However, as the general type of the formations of Europe cannot be modelled after that of a single district, the necessity of admitting parallel formations (sich vertretende Gebirgsarten), renders the names of first or second gypsum, of old or middle sandstone, extremely vague and obscure.

In one country it is proper to consider a bed of gypsum or of common sandstone as a particular formation; while in another, it should be regarded as subordinate to neighbouring formations.

The geographical denominations are without doubt the best; they give rise to precise ideas of superposition.

When it is said that a formation is identical with the porphyry of Christiania, the lias of Dorsetshire, the sandstone of Nebra (bunter sandstein), the coarse limestone of Paris, these assertions leave no doubt in the mind of an experienced geognost, regarding the position which is to be assigned to the formation in question.

It is also by silent convention, as it were, that the words zechstein of Thuringia, Derbyshire Limestone, Paris Formation, &c. have been introduced into mineralogical language.

They express a limestone which immediately succeeds the red sandstone of the coal deposite, a transition limestone placed beneath the coal sandstone, and lastly formations of more recent origin than the chalk.

The only difficulties which the multiplicity of these geographical denominations presents, consist in the choice of names, and in the degree of certainty which may have been acquired, regarding the position or relative age of the rock to which the others are referred.

The English geognosts look upon the continent for their lias and red marl; the German for their bunte sandstein and muschelkalk.

These words present themselves in the minds of travellers associated with remembrances of localities.

It is not of so much importance, therefore, to produce precise ideas, as to make choice of localities generally known, and which are celebrated, either by the working of mines, or by descriptive works.

In order to diminish the effects of national vanity, and to attach new names to more important objects, I proposed a long time ago, (1795), the denominations of Alpine Limestone, and Jura Limestone.

A portion of the High Alps of Switzerland, and the greater part of Jura, are without doubt formed of these two rocks: the names, however, generally received at the present day, of Alpine Limestone (Zechstein), and Jura Limestone, should in my opinion be modified or entirely abandoned.

The lower beds of the Jura mountains, filled with gryphites, belong to an older formation, perhaps to the zechstein; and a great part of the limestone of the Alps of Switzerland assuredly is not zechstein; but, according to Messrs de Buch and Escher, transition limestone.

It would therefore be better to choose the geographical names of rocks from among the names of isolated mountains, the whole visible mass of which belongs only to a single formation, than to derive them, as I have erroneously done, from entire chains.

I have thought, and many geognosts have formed the same opinion, that the Jura limestone (cavernous limestone of Franconia) was generally placed upon the continent, beneath the Nebra sandstone, (bunte sandstein), between this sandstone and the zechstein.

Subsequent observations have proved, that the name of Jura Limestone had with reason been applied to rocks which are very distant from the mountains of Western Switzerland; but that the true geognostical place of this formation, (when there is not a suppression of the inferior formations), occurs above the Nebra sandstone, between the shell-limestone (muschelkalk, or the quadersandstein), and the chalk.

A geographical name, justly applied to several analogous rocks, renders us attentive to their identity of relative position; but the place which homonymous rocks ought to occupy in the total series, is not well determined except when the geographical name has been selected, after having acquired a perfect certainty regarding their position.

Circumstances are the same with regard to the relative age of the molasse of Argovia (nagelfluhe) and of the Pirna quadersandstein (gres blanc of M. de Bonnard), two rocks of very recent origin, which have been very well examined separately, but whose relations to each other, and to the chalk and Jura limestone, have only been illustrated of late.

One may therefore be pretty sure of having met in the New Continent with rocks identical with the molasse or quadersandstein, without being able to pronounce with certainty upon their relations with all the secondary or tertiary rocks.

When rocks are not immediately in contact, and are not covered by deposites of known position, their relative age can only be conjectured from simple analogies.

The terms of the geognostical series are either simple or complex.

To the simple terms belong the greater number of the primitive formations: the granites, gneisses, mica-slates, clayslates, &c. The complex terms occur in greater numbers among the transition rocks: there, each formation includes an entire group of rocks, which alternate periodically.

The terms of the series are not transition limestones or greywackes, constituting independent formations; they are associations of clayslate, greenstone, and greywacke; of porphyry and greywacke; of granular steatitic limestone, and of conglomerates, composed of primitive rocks; of clay-slate and black limestone.

When these associations are formed of three or four rocks which alternate, it is difficult to give them significant names,--names indicative of the whole composition of the group,--of all the partial members of the complex term of the series.

It may then assist in fixing the groups in the memory, to retrace the rocks which predominate in them, without being absolutely wanting in the neighbouring groups.

It is in this manner that the granular steatitic limestone characterises the Tarantaise formation; the greywacke, the great transition formation of the Hartz and of the banks of the Rhine; the metalliferous porphyries rich in hornblende, and almost destitute of quartz, the formation of Mexico and of Hungary.

If these phenomena of alternation attain their maximum in the transition districts, still they are not entirely excluded from the primitive and secondary terrain.

In both of these terrains, complex terms are mixed with the simple terms of the geognostical series.

I shall mention among the secondary formations, the sandstone placed below the alpine limestone, (the Nebra sandstone, bunte sandstein), which is an association of marly clay, sandstone and oolites; the limestone which covers the red-sandstone of the coal-formation (the zechstein or alpen-kalkstein), which is a less constant association of limestone, of (muriatiferous) gypsum, of stinkstone and of pulverulent bituminous marl.

In the primitive class we find the three first terms of the series; the oldest rocks either isolated, or alternating two and two, according as they are geognostically more approximated by their relative age, or the whole three alternating.

The granite sometimes forms constant associations with the gneiss, and the gneiss with the mica-slate.

These alternations follow particular laws: we see, (for example in Brazil, and, although less distinctly, in the maritime chain of Venezuela), the granite, gneiss and mica schist in a triple association; but I have not found granite alternating alone with mica-slate, or gneiss and mica-slate alternating by themselves with clay-slate.

We must not confound, and on this point I have often insisted in the present article, rocks passing insensibly to those which are in immediate contact with them; for example, micaslates, which oscillate between gneiss and clay-slate, with rocks which alternate with one another, and which preserve all their distinctive characters of composition and of structure. M. D'Aubuisson has long ago shewn how chemical analysis approximates the clay-slate to mica.

(Journal de Physique, vol. 68. p. 128.; Traite de Geognosie, vol. ii. p. 97.)

The first, it is true, has not the metallic lustre of mica-slate; it contains a little less potash, and more carbon; the silex does not unite into nodules or thin laminae of quartz, as in the mica-slate; but it cannot be doubted, that scales of mica form the principal base of the clay-slate.

These scales are so joined together, that the eye cannot distinguish them in the mass.

It is perhaps this same affinity which prevents the alternation of clay-slates and mica-slates: for in these alternations Nature seems to favour the association of heterogeneous rocks; or, to make use of a figurative expression, she delights in the associations whose alternating rocks present a great contrast of crystallization, of mixture and of colour.

At Mexico, I have seen dark greenstones, alternate thousands of times with reddish-white syenites, abounding more in quartz than in felspar.

In this greenstone there were veins of syenite, and in the syenite veins of greenstone; but none of the two rocks passed into the other.

(Essai politique sur la Nouvelle Espagne, v. ii. p. 523.)

They present at the limit of their mutual contact, differences as marked as the porphyries which alternate with the greywackes or with the syenites, as the black limestones which alternate with the transition clay-slates, and so many other rocks of entirely heterogeneous composition and aspect.

Further, when, in primitive deposites, rocks more related by the nature of their composition than by their structure or mode of aggregation, for example, the granites and gneisses, or the gneisses and mica-schists, alternate; these rocks do not by any means show the same tendency to pass into each other, as they present, when isolated in formations which are not of a complex character.

We have already observed, that often a bed b, becoming more frequent in the rock a, announces to the traveller that the simple formation a, is to be succeeded by a compound formation, in which a and b alternate.

Farther on, it happens, that b assumes a greater development; that a is no longer an alternating rock, but a simple bed subordinate to b, and that this rock b shows itself alone, until, by the frequent repetition of beds g, it becomes the precursor of a compound formation of b, alternating with g.

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).

Pasigraphic language has the advantage of generalizing the problems; it is more conformable to the wants of geognostical philosophy, of which I attempt to present here the first elements, in so far as they have relation to the study of the superposition of rocks.

Now, if often between formations which are simple and very closely allied, in the order of their relative antiquity, between the formations a, b, g, there occur compound formations interposed, ab and bg, (that is to say a alternating with b, and b alternating with g); we observe, also, although less frequently, that a formation (for example a,) assumes so extraordinary an increase, that it envelopes the formation b; and that b, instead of showing itself as an independent rock, placed between a and g, is now nothing but a bed in a. It is thus, that, in Lower Silesia, the red-sandstone contains the formation of zechstein; for the limestone of Runzendorf, filled with impressions of fishes, and analogous to the bituminous marl abounding in fishes of Thuringia, 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 phenomenon in Hungary.

In other districts, for example, in Switzerland, at the southern extremity of Saxony, the red-sandstone disappears entirely; because it is replaced, and, so to speak, overcome, by a prodigious development of greywacke or of alpine limestone.

(Freiesleben, Kupfersch. p. 109.)

These effects of the alternation or unequal development of rocks, are so much the more worthy of attention, that their study may throw light upon some apparent deviations from a generally acknowledged type of superposition, and that it may serve to refer to a common type the series of position observed in very distant countries.

In order to designate the formations composed of two rocks which alternate with another, I have generally preferred the words granite and gneiss, syenite and greenstone, to the more commonly adopted expressions of granite-gneiss, syenitegreenstone.

I was apprehensive that this last method of designating formations composed of alternating rocks, might rather give rise to the idea of a passage from granite to gneiss, from syenite to greenstone.

In fact, a geognost, whose works upon the trachytes of Germany have not been sufficiently appreciated, M. Nose, has already made use of the words granite-porphyries and porphyry-granites, to indicate varieties of structure and aspect, 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 adopting the denominations of granite and gneiss, of syenite and porphyry, of greywacke and porphyry, of limestone and clay-slate, no doubt is left regarding the nature of the complex terms of the geognostical series .

Translated from Essai Geognostique par Baron Alexandre de Humboldt.

M. Humboldt next proceeds to consider the natural history of fossil organic remains, as connected with formations.

This very interesting paper on petrifactions, has already appeared in No. xvii. of our Journal.