« PrécédentContinuer »
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 foetz 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 hereaster.* 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 sys. tem, 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 minerals which are thus altered are porcelain, jasper, earth, slag, burnt-clay, columnar clay, ironstone, and, perhaps, polishing slate.
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
• See series II. lect. ii. On zoological systems, and the distinctive characters of animals.
VEINS, as if the rocks they compose had split asunder in different places from top to bottom, and the chasms had been afterward filled up from other
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, greenstone, 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 cr vein always separating and vanishing at the point of intersection.
Nothing appears more obvious than that these veins must have been origi. nally 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 difserent 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 appearance. They are supposed to exist over the globe generally, and to be independent of chorographic or typographic changes, and have hence been still farther denominated UNIVERSAL FORMATIONS. ·
M. Werner has since, however, been induced to add to these a sixth class, consisting of what he has called Partial or LOCAL FORMATIONS: comprising those which are so often found in vast hollows or basins of particular 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 fresh water 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 Æningen, 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 fresh water; 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 are 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 rolled or violently distorted, or deprived of their natural contour, such remains have not been brought to their present beds from a distance; but that the deluge must have been sudden, and overtaken them in their natural resorts; and hence may, in many cases, have swept away all the individuals of a species in a common calamity.
There is, however, a great difficulty with some naturalists in conceiving that such animals as the elephant, the tapir, the rhinoceros, the hippopotamus, the mammoth, or mastodon, animals now only found in the torrid regions, could have existed in these northern parts of the globe. M. de Marschall endeavoured by one sweeping stroke of the fancy to solve this, as well as that of the extraordinary fragments in which they are often imbedded, and held out that the whole have fallen at different times, like meteoric stones, from heaven. The real difficulty, however, vanishes in a considerable de gree, if not entirely, when we reflect, that although the torrid regions furnish us with some of these genera, they do not appear in any instance to contain the same precise species as are traced among the large fossil quadrupeds of the northern and colder parts: and hence it is no argument, that because the habits of the extant species do not qualify them for a residence in these latter regions, such situations might not have furnished a comfortable home to the species whose remains are found among us. The fossil species do not differ less from the living to which they make the nearest approach, than various animals that are familiar to us do from others that belong to the same tribes, and which are found, under one species or other, over the whole world. The race of horses, of swine, or of sheep, furnishes us with abundant examples of this remark: and that of dogs affords perhaps a still more striking illustration; for while under one form, that of the isatis or Arctic fox, the canis Lagopus of Linnæus, we find it in the northernmost coast of America, and even the frozen sea, living in clefts, or burrowing on the naked moun. tains, and in that of the almost infinite varieties of the c. familiaris or domestic dog, in the bosom of our own country,—in the form of the c. aureus, chacal or jackal, we meet with it in the warmest parts of Asia and Barbary, prowling at night in flocks of one or two hundred individuals.
* For an admirable defence of this part of the theory, sw Mr. Jameson's essay “ On Formations,” inverted in the Annals of Philos. No. iii. p. 191.
| Phil. Trans. for 1813, p. 135. See alsn Mr. Webster's valuable essay on the same subject, in vol. ii. of the Transactions of the Geological Society.
Recherches sur l'Origine, &c. Geissen, 1802.
The extensive TURBARIES or PEAT-FIELDS, which are so common to many parts of Europe, are produced by an accumulation of the remains of sphagnum and other aquatic mosses. These surround and cover up the small knolls upon which they are formed; or, in many places, descend along the valleys after the manner of the glaciers of Switzerland; but, while the latter melt away every year at their lower edges, the mosses are not checked by any obstacle in their regular increase; and as such increase takes place in determinate proportions, by sounding their depth to the solid ground we may form some estimate of their antiquity.
The ordinary rise of those extensive ranges of downs which are seen skirting the coasts of many countries, and especially where the shore is not very bold, is a mixed effort of sea and wind. To produce this, however, the soil that the sea washes over must consist of sand. This is first pushed in successive tides towards the shore; it next becomes dry, by being left there at every reflux of the sea; and is then drifted up the beach, and to a considerable distance from the beach, by the winds which are almost always blowing from the sea, and often in whirls or eddies; and are at length fixed by the growth of wild plants, whose seeds are in like manner wasted about on the wings of the breeze, or casually dropped with the excretions of birds or other animals that pass over them. In several parts, observes. M. Cuvier, these proceed with a frightful rapidity, overwhelming forests, houses, and cultivated fields in their irresistible progress. Those on the coast of the Bay of Biscay have actually buried a considerable number of villages whose existence is noticed in the records of the middle ages. And even in the present day they are threatening not fewer than ten distinct hamlets with almost inevitable destruction : one of which, named Mimigan, has been in perpetual danger for upwards of twenty years, from a sand-hill of more than sixty feet in perpendicular height, produced by the cause we are now contemplating, and which is very obviously augmenting.*
There are various forelands on the coasts of the North Sea, and particularly on those of the counties of Sleswigh and Holstein, which are formed in the same manner.f But the most extraordinary inroads of sand storms and sand floods are, perhaps, those which have taken place in the Libyan Desert and in Lower Egypt. M. Denon informs us, in his travels over this part of the world, that the summits of the ruins of ancient cities buried under mountains of drifted sands still appear externally; and that but for a ridge of mountains, called the Libyan Chain, which borders the left bank of the Nile, and forms a barrier against the invasion of these sands, the shores of the river, on that side, would long since have ceased to be habitable. “Nothing,” says M. Denon, can be more melancholy, than to walk over villages swallowed by the sand of the desert, to trample under foot the roofs of their houses, to strike against the tops of their minarets, and to reflect, that yonder, in days of yore, were cultivated fields, that hard by were groves of flourishing trees, and the dwellings of men close at hand;—and that all has now vanished.”I
The various Islands that spot the surface of the sea have arisen from different causes. Many of them have been merely separated from the adjoining continent by the inroad of the sea itself upon the mainland ; others have been thrown up by volcanoes, which have at times disgorged prodigious blocks of granite among the mixed materials, such as are frequently found in the Danish archipelago, in the midst of the geest, or alluvial matter, which has collected around them. . Other islands are altogether the masonry of madrepores, and other coral zoophytes of wonderful industry and perseverance, of which the South Sea furnishes us with the largest and most astonishing specimens. These islands are for the most part flat and low, and surrounded by enormous belts of coral reefs. Most of the calcareous zoophytes are employed in their construction, but the principal worm is the madrepora lubricata of Linnæus.
* Report concerning the downs of the Gulf of Gascony, or Bay of Biscay, by M. Tassin, Mont de Mar san. an. x. Cuvier, Theory of the Earth, 31.
| De Luc, Voyages Géologiques, tom. 1. Jamoson's Notes on Cuvier's Theory, &c. p. 217. Compare Dolomieu's Memoir on Egypt, iu Journ. de Physique, tom. xlii.
In so large an abundance, and with so much facility, is calcareous matter elaborated by these, as well as by various other animals, and especially the testaceous worms, that M. Cuvier is inclined to ascribe all the calcareous rocks that enter into the solid crust of the earth to an animal origin.* But this is to suppose the earth of a far higher antiquity, and to have been the subject of more numerous general deluges, and inversions of sea and land, than are called for by the Wernerian system, or appear reconcileable with the Mosaic narrative. M. Cuvier apprehends, indeed, that such catastrophes may have occurred five or six times in succession, at a distance of four, five, or six thousand years from each other; and that even the chalk formation found in the basin of Paris originated in a revolution of this kind that occurred antecedently to that which is usually regarded as the flood of Noah. And, following up this idea, he conceives, towards the close of his Introductory Theory of the Earth, that if the science of fossil organic productions could be carried to a much higher degree of perfection, we should be able to obtain far fuller information upon this subject ; " and man, to whom only a short space of time is allotted upon the earth, would have the glory of restoring the history of thousands of ages which preceded the existence of the human race, and of thousands of animals that never were contemporaneous with his species."
(The subject continued.) In our last study I attempted a brief sketch of the chief phenomena that occur to the eye of the geologist upon a survey of the solid crust of the earth, as far as he is able to penetrate into it. The conclusion to which such phenomena lead us is the following: that the rudimental materials of the globe, to the utmost depths we are able to trace them, existed at its earliest period, in one confused and liquid mass; that they were afterward separated, and arranged by a progressive series of operations, and a uniform system of laws, the more obvious of which appear to be those of gravity and crystallization; and that they have since been convulsed and dislocated by some dreadful commotion and inundation that have extended to every region, and again thrown a great part of the organic and inorganic creation into a promiscuous jumble.
Now, the only two causes that can enter into the mind of man as being competent to the fluidity that apppears at first to have existed throughout the whole crust of the earth are Fire, or a peculiar soLVENT. But, if a solvent, that solvent must have been water: for there is no other liquid in nature in sufficient abundance to act the part of a solvent upon a scale so extensive.
And hence our inquiries into this subject become in some degree limited, and are chiefly confined to what have been called the Plutonic and the NepTUNIAN hypotheses; the origin of the world in its present state from igneous fusion, and from aqueous solution. Both these theories are of very early
Some writers have proceeded much further than this, for they have resolved all the solid materials of the earth's crust into aii organic origin. Such was the opinion of Demaillet and Lamarck, who suppose that every thing was originally fluid'; that this universal fluid gave rise to plants and animals; that all clay or argillaceous earth is the produce of the former; all calcareous earth of the latter; and that siliceous earth has been the result of the two. Telliamid, p. 169. Philosophie Zoologiquo, passim