Imágenes de páginas
PDF
EPUB

own inequality of thickness in different places, and partly from other causes, the continuity is often interrupted-yet still we trace enough of it to convince us that the rocks which constitute the crust of the earth, when contemplated upon a large scale, are every where the same, and that they invariably occupy a like situation with respect to each other.

The labours of Mr. Kirwan and M. de Saussure gave the earliest hints upon this subject; and the geological theories of Professor Werner of Freyburg, and of M. de Cuvier of Paris, are entirely founded on the same. These theories, though derived in some measure from different sources of mineralogical study, coincide not merely in their general outline, but in all their more prominent parts, and only differ in their mode of accounting for the more limited or local deposites.

M. Werner," from whom alone," to adopt the language of M. de Cuvier, "we can date the commencement of real geology," so far as respects the mineral natures of the strata, divided, in his first view of the subject, all the various rocks that enter into the solid crust of the earth into FIVE classes.

Of these the FIRST CLASS consists of those rocks which, if we were to suppose each layer to be extended over the whole earth, would lie lowest, or nearest the centre, and be covered by all the rest; it comprises seven distinct sets, as granite, gneiss, mica-slate, clay-slate, a peculiar kind of porphyry, sienite, and a peculiar kind of serpentine. Of these, granite lies the undermost, and sienite the uppermost; and in the midst of several of them we meet with beds of not less than eight other kinds of rock, as though dropped into them by accident-as topaz, another kind of porphyry, serpentine, limestone, flint-slate, and trap, quartz, and gypsum; which are hence called subordinate rocks of this class, and which extend the whole number of sets belonging to it to fifteen.

These are supposed to have been earliest produced, and when the earth first emerged from a state of chaos to a state of order; and are hence denominated PRIMITIVE FORMATIONS. They are distinguished by the following character. Not a single relic of either animal or vegetable petrifaction is to be found in any of them. The lowermost or older contain no carbonaceous matter; which is discoverable but very sparingly in the superior or newer. They are all chemical combinations, and generally crystallized; the crystallized appearance being most perfect in the oldest, and gradually becoming less perfect in the newer formations. I have already observed that the whole of this scale of formation does not regularly coat the nucleus of the earth; so little so, indeed, that sometimes even the granite itself, the lowermost rock of all, is left bare, and not pressed down or coated by a deposite of any other kind of rock and so of the rest. Wherever this deficiency takes place, the rock thus left at liberty rises uniformly higher than it is found to do where pressed upon and invested with its common coatings. But every rock does not, under such circumstances, rise equally high, or with an equal degree of freedom; for granite rises highest of all: and hence we frequently find it composing the tops of our loftiest chains of mountains, as well as the basis of the earth's solid crust. It forms the great body of the Swiss mountains and the Alps, though gneiss is here also found in great abundance.

The level of gneiss, when left at equal liberty, is a little lower than that of granite. It constitutes the vast mass of the Carpathian mountains that divide Transylvania and Hungary from Poland.

The level of mica-slate is lower than that of gneiss, and the level of clay-slate lowest of all. So that there is a regular sinking of these re

spective levels from granite to clay-slate: while the newer porphyry and sienite are often laid over their summits, as though these two formations had been deposited long after the production of the others; an idea which is still further strengthened by our meeting occasionally with a bed of breccia, or pudding-stone, composed of fragments of the older or lower rocks, capping the gneiss, granite, or other formation before the porphyry or sienite has been deposited.

The SECOND CLASS of rocks, or that which, when the number of coatings is complete, lies immediately over the preceding, consists of gray-wacke slate, and a peculiar kind of limestone, green-stone, and amygdaloid; together with subordinate masses of the proper primitive formations, sienite, porphyry, and granite: as though some portions of these had become crystallized after the rest, along with the next layers in succession, or had been separated from the parent rocks by some early commotion. Gray-wacke, which is a concrete term, denoting a conglomerate rock of a peculiar kind having a basis of clay-slate, and being studded or otherwise intersected with portions of quartz, felspar, and scales of mica, may be exemplified by what in Cornwall is called killas, a far more euphonous word; and hence gray-wacke and gray-wacke slate may be distinguished by the terms amorphose and schistose killas. The Cornish killas lies directly over the granite of that country, which possesses the character ascribed by Werner to granite of the highest antiquity.*

These formations, for the most part, irregularly alternate with each other, instead of preserving one regular and successive order, as the dif ferent sets of the primitive formations do; excepting that the limestone appears usually undermost, and placed, as the basis of the rest, upon the sienite or uppermost of the first class. It is in this second class of forma tions that petrifactions first make their appearance; and it deserves particular attention that they are uniformly confined, both in the animal and vegetable kingdoms, to those of the lowest links in the scale of organization; and even among these to species which are at present altogether unknown, and which appear therefore to be totally extinct. Thus the animal petrifactions consist entirely of ammonites, mytilites, unknown corals, and other zoophytic worms; and the vegetable petrifactions of reeds, ferns, and other palm-like plants, mosses, and other cryptogamic productions, which occupy the lowest part in the scale of vegetable life, as zoophytic worms do among animals. It is here, also, that carbonaceous matter, which is chiefly of vegetable origin, first makes its appearance in any considerable quantity.

To this class of rocks, therefore, M. Werner has given the name of TRANSITION FORMATIONS; as believing them to have been produced while the earth was in a state of transition from inorganic matter to organic life, -from an uninhabited to an inhabited condition. The date of their formation, however, is proved, even from their natural appearance, to have been very remote; since, as already observed, the whole of the petrifactions which they contain consist of plants and animals, not only of the very lowest species, but which now seem to be altogether extinct.

The THIRD CLASS of rocks is denominated FLOETZ, that is, FLAT or HORIZONTAL, FORMATIONS, in consequence of their usually appearing in beds much more nearly horizontal than the preceding. They lie imme

See Allan's remarks on the transition-rocks of Werner, in Thomson's Annals of Philos vol. ii. p. 23. Compare with Jameson's definition of the same. Id. Feb. 1817, p. 17.

diately over the transition-class, and consist of the twelve following distinct sets of rock, each of which is generally found in a particular situation : sandstone of different kinds, and differently arranged, three sets; limestone, three sets; gypsum, two sets; calamine; chalk; coal; trap. The trap usually covers the whole of this class, as the newer porphyry and sienite cover the primitive formations: the relative position of the rest is more variable. The floetz or horizontal class is characterized by its containing an abundance of petrifactions in every one of its sets, and these of known animal and vegetable kinds; though still, of those that occupy the lower parts of the scale, as shells, fishes, the fishes much mutilated, a few tortoises, ferns, pines, and reeds; indicating that they were formed at a period in which organized beings of this character abounded, but in which those of other characters did not exist, or but rarely.

The FOURTH CLASS of formations, under the Wernerian system, is denominated ALLUVIAL, and constitutes the great mass of the actual surface of the earth's solid crust. They have been evidently produced by the gradual action of rain, river-water, air, and the elastic gases, upon the other classes, and may, comparatively, be considered as very recent formations, or rather as deposites, whose formations are still proceeding. They may be divided into two kinds; those deposited in the valleys of mountainous districts, or those elevated plains which often occur in mountains; and those deposited upon flat land.

The first kind consists of sand, gravel, and similar materials, which constituted part of the neighbouring mountains in their original state, and which remain, notwithstanding that these less durable parts have been thus washed or blown away. They sometimes contain ores, which also existed in the neighbouring mountains, and have been carried down by the agency of rain, air, or the elastic gases. The ores principally discovered in such situations are those of gold and tin; and these soils are often washed in order to separate them. Beds of loam are also occasionally met with on the plains of mountains, formed of the decomposed elements of animal and vegetable bodies that once occupied their sides.

The second kind of alluvial deposites, or that which occupies the flat land, consists of loam, clay, sand, marl, calcsinter, and calctuff, or stalactitic tufa, the basis of our common petrifactions; and which is found very largely in Sweden, Germany, and Italy, clothing with a calcareous coat the smaller branches of trees, leaves, prickles, moss, and other minute plants; eggs, birds, and birds-nests; preserving them from decay, by defending them from the action of the air. The clay and sand sometimes contain petrified wood; and in many parts are found the skeletons of quadrupeds, even of the largest magnitudes, as we shall have occasion to observe hereafter.* Here, also, occur earths and brown coal (in which is often traced mineral amber), wood-coal, bituminous wood, and bog iron

ore.

The LAST, or UPPERMOST, of THE FIVE CLASSES of rocks of the Wernerian system, is denominated voLCANIC FORMATIONS; and consists of two distinct sets, false and true.

The false comprise mineral substances which have experienced a change from the combustion of beds of coal situated in the neighbourhood: the chief materials which are thus altered are porcelain, jasper earth, slag, burnt-clay, columnar clay iron-stone, and, perhaps, polishing slate.

* See Series II, Lect. II. On zoological systems, and the distinctive characters of animals.

The real volcanic minerals are those which have been thrown out of the crater of a volcano, and consist of three kinds: first, those which, having been discharged frequently, have formed the crater itself of the mountain: secondly, those which have rolled down in a stream, and are known by the name of lavas: and, thirdly, the residual matter contained in the water which is often ejected, composed of ashes and other light substances, and which, when rendered solid by evaporation, is denominated volcanic tuff or tufa.

I have observed, that these different classes of mineral formations are often traversed in various directions by other mineral substances which are called VEINS, as if the rocks they compose had split asunder in different places from top to bottom, and the chasms had been afterwards filled up from other sources. These transverse lines or veins are worthy of notice in regard to their shape and the substances with which they are filled.

With respect to their shape, they appear to be almost always widest above, and gradually to diminish as they deepen, till at last they terminate in a point; exactly as if they had been originally fissures in the rock. Occasionally, indeed, they are observed to widen and contract alternately in different parts of their course, but this is by no means a common appearance.

Sometimes they are partially or altogether empty; and in this case they are real fissures, and are so denominated; but generally they are filled with matter more or less simple, and more or less different from the rock through which they pass. All the formations I have already noticed as existing in the shape of rocks have also been found in the shape of veins = whence we have veins of granite, porphyry, limestone, basalt, wacke, green-stone, quartz, clay, felspar, pit-coal, common salt, and metals of every kind. When the veins are compound, or consist of a variety of substances, these substances are almost always disposed in regular layers; one species of mineral constituting a central line or cylinder, and this being incrusted with a second mineral, and the second with a third, and in the same manner to the utmost sides of the veins. These layers are occasionally very numerous; that of the vein Georgius at Freyburg consists of not less than nine, and there is another in the same district, which, according to M. Werner, extends to thirteen. It is not uncommon to find veins crossing each other in the same rock; and when this occurs, one of the veins may be traced passing through the other without any interruption, and completely cutting it in two, the cut vein always separating and vanishing at the point of intersection.

Nothing appears more obvious than that these veins must have been originally fissures produced by some unknown violence in the rocks in which they occur; and it is highly probable, as conjectured by M. Werner, that the mineral materials which constitute them have been deposited slowly from above during the formation of the different classes or sets of rock of which the different layers consist, while the rocks in which they occur were covered with water. Upon this theory veins are of course newer than the rocks in which they are met with, and which must have split to have produced them; and where two veins cross each other, that is obviously the newest that traverses the adjoining without interruption, as the fissures constituting the second vein must have been formed after the first was filled up.

The FIVE classes of rock formations we have thus far considered are those which entered into Professor Werner's system, as it first made its

[ocr errors]

1

appearance. They are supposed to exist over the globe generally, and to be independent of chorographic or topographic changes, and have hence been still further denominated UNIVERSAL FORMATIONS.

M. Werner has since, however, been induced to add to these a SIXTH class, consisting of what he has called PARTIAL OF LOCAL FORMATIONS : comprising those which are so often found in vast hollows or basins of par ticular countries; the materials of which are, in many instances, strangely intermixed, and have probably been carried down into such basins by circumscribed deluges, produced by an exundation of rivers or seas, occasionally alternating with each other, or by other partial disruptions. We have here, therefore, reason to expect,-what in fact is perpetually met with,-a motley combination of whatever substances may have existed in the course of such seas or rivers or rifted soils, with masses or fragments of most of the UNIVERSAL FORMATIONS, alternate beds of marine and freshwater alluvions, and, consequently, animal and vegetable remains of all kinds.

The composite rocks that fill up the great basin around Paris, in which the skeletons of so many unknown animals, even quadrupeds of the hugest size, elephants, hippopotami, tapirs, mammoths, and other pachydermatous, or thick-skinned monsters, have been discovered, are of this LOCAL FORMATION. The celebrated quarries of Eningen, on the Rhine, are of a like kind; and these, having been erroneously regarded of the same antiquity as Werner's UNIVERSAL FORMATIONS, have been appealed to by various writers as affording proofs of the falsity of his theory.*

We have other instances of this local formation in many parts of our own country, and particularly near the banks of the Thames. Mr. Trimmer has given an interesting account of the substrate of two fields in the vicinity of Brentford, that are loaded with the organic remains of the larger kinds of quadrupeds; as bones of elephants, approaching to both the Asiatic and the African species; horns of deer, apparently as enormous as those dug up in Ireland; bones of the bos genus; and teeth and bones of the hippopotamus; the last very abundant, and intermixed with fresh water shells, and other fresh-water relics.

Occasionally, however, marine remains are found intermingled with such animal fossils, and composing their beds instead of those of freshwater; and not unfrequently layers of the one kind, as in the basin of Paris, are irregularly surmounted by layers of the other. But no human skeletons are discovered in the midst of any of these rocks, although the bones of man are as capable of preservation as those of any other animal : the only known instance of this sort being that imported into our own country from Guadaloupe by Sir Alexander Cochrane, and which is now exhibited in the British Museum, imbedded in a block of calcareous stone; a very accurate description of which has been published in the Philosophical Transactions by Mr. König.

It is hence obvious, that the catastrophes which involved these enormous quadrupeds in destruction must have occurred at a period when mankind had no existence in the regions which were thus overwhelmed; and in some places overwhelmed alternately by disruptions and inundations of sea and of fresh water. And it is equally obvious, that as the fossil bones are not

*For an admirable defence of this part of the theory, see Mr. Jameson's essay "On Formations," inserted in the Annals of Philos. no. iii. p. 191.

† Phil. Trans. for 1813, p. 135. See also Mr. Webster's valuable essay on the same subject, in vol. ii, of the Transactions of the Geological Society.

« AnteriorContinuar »