XI. BOOK conglobated beds has given rise, let us proceed to the examination ‹f stratified rocks. In order to shew how vague are the limits between the productions of pure crystallizaStratified tion and those of stratification, let us be permitted to have recourse for a moment to an hypothesis borrowed from chemistry. rocks. Chemical and mechanical solution. The elements of which the rocks are formed, were undoubtedly once in a fluid state; this condition is necessary to render the aggregation of so many different substances possible. But the state of solution in which these substances existed, presented two modifications. Chemical solution differs essentially from mechanical solution; the latter exists when the integral particles of a body are separated one from the other, and suspended in a fluid; on being deposited according to their specific gravity, they form a sediment; on the contrary, by chemical solution, the integral particles are not only separated from one another, but are combined with the solvents; and in uniting, according to a new development of affinities, independently of their specific gravity, they afford a precipitate. Every crystallized substance is formed by chemical solution, and by precipitation; this character belongs, in the whole force of the term, to primordial rocks; but we have seen how crystallization lost by degrees its simplicity, and, if we may use the expression, its primitive energy. have also seen how the crystallized texture has been succeeded by the foliated and the compact structure, which, in becoming larger and larger, seems to indicate a regular scale of chemical precipitations, gradually approaching to the nature of mechanical sediments. There can be no doubt, that the most immense of all fluids, the ocean, suffered a gradual decrease of its waters; the primordial mountains first reared their summits, then the earth began to swell up out of its chaotic state. Organic beings were then in existence; these were marine animals. The chemical precipitation continued, but, at the same time, the first mechanical sediments began to be deposited; the two precipitates were mixed, sometimes in great confusion, together. XI. Thus arose the rocks of transition, which are found on BOOK the sides of the primordial mountains, and sometimes at their base, in which we observe the first animal remains. Rocks of The waters having gradually subsided, the globe from a transition. shapeless chaos, takes the appearance of one vast sea, from which there arose on all sides, the primordial mountains, flanked by rocks of transition. This is the Neptunian age, the second geological era. The sediments of the mechanical solutions alternated with the chemical precipitations; the solvent having gradually lost its original power, chemical action insensibly gave way to that which was purely mechanical. This Neptunian age gave birth to stratified mountains and rocks, understanding Stratified by the word stratum a bed of homogenous substance. This structure. term, borrowed from the Latin, is not then precisely synonymous with bed or layer; a stratum differs still more from a lamina or plate, terms made use of in speaking of the texture, that the substance of certain rocks present, even in their minutest fragments. rocks. The hypothesis we have just now traced, exhibits the difficulties which geologists have met with in their attempts to classify stratiform rocks. In the calcareous rocks espe- Stratified cially, the compact structure and the stratified are blended calcareous together by innumerable shades. Even in the same rocks, and notwithstanding its being completely homogeneous, we find the upper part containing petrifactions belonging to marine animals, while the lower beds are entirely free from them; but amongst the decidedly stratified rocks, we should place nearly all the aggregates of sulphate and carbonate of lime, known under the name of lime-stone; then the secondary marbles, which owe their earthy fracture to large quantities of argil, and their colours so agreeably varied to the presence of oxide of iron; ruiniform marble, Marbles which, according to Dolomieu, was a calcareous argillaceous ed, shelly, stone, uniformly coloured, but while drying cracked into &c. fissures; and these, filled by the transuding of the calcareous matter, have formed the appearances of ruins and ruiniform XI. Chalk. BOOK landscapes, which are so much admired ;* lastly, the shelly marble, composed of an infinity of small shells united by a calcareous cement. The fœtid stone, or swine stone, which in Norway forms of itself a little island, is only carbonate of lime, penetrated by bitumen. Calcareous substances predominate in the stratified rocks; the capital of France, with its palaces and temples, is built of calcareous stone, almost entirely composed of two kinds of shells: the cerites, (cyrithium,) which forms the upper masses, and the miliolites, (miliola,) which abounds in the deepest beds. These rocks readily indicate the rank which belongs to them; but what rank can be assigned to the calcareous stone which presents itself under the form of chalk, and which occupies such vast belts throughout the globe? From its purity, it is almost a simple substance, containing only lime, carbonic acid, and phosphoric acid. From the small number of marine animals which it immediately contains, it resembles the primitive rocks from which its earthy consistence, destitute of the least appearance of crystallization, seems to keep it perfectly distinct; finally, the layers of large flint so frequently met with amongst beds of chalk, clearly indicate that they have been formed by deposit, and these beds inclose a great many marine animals of species which no longer exist. It appears that the chalky state of the calcareous matter is owing to some particular causes, to investigate which is the province of chemistry. Argile, or clay. Aluminous earth enters into a great number of stratified substances; clay, which occupies great spaces on our globe, is properly a mixture of silex and alumina, modified by the accidental presence of magnesia, iron, and other substances. Clay is found in mounds and in rocks; it seems natural to us to suppose in general, that these last may be formed by the hardening, or rather by the confused crystallization of argillaceous earths, mixed sometimes with grains of quartz. * Dolomieu, Journal de Physique, Octobre 1793, p. 285, et suiv. † Lamarck, Mémoires sur les Fossiles des environs de Paris. Annal du Muséum d'Hist. Natur. Comp. Brongniart, Traité de Minéralogie, i. 208-210.\ XI. But argillaceous earths may be formed also by the decom- BOOK position of rocks; such would be, according to Werner, the origin of potters' earth, which he places among the ternary earths or those of alluvion. All argillaceous soils have, without doubt, often changed their situation, their layers gliding easily one over another; their principal situation, however, is in the secondary rocks; they are found alternately with the free-stone, sand, and shell limestone. Lithomarge, distinguished by its feeling greasy, and adhering to the tongue, by the fineness of its grain, and by its fusibility into a spungy mass, is the only clay which is found in the clefts of rocks in primitive formations. This is the substance to which several nations of America, Africa, and Siberia, have recourse to appease their hunger, or rather to beguile for a moment the cravings of their digestive organs,* Amongst the other kinds of clay, we distinguish for their utility those employed in the operations of the potter and the fuller, the different boles and ochres; the slate, which is of such service in roofing houses, and which Slate. is only indurated clay, in the form of schist or in laminæ. Slate, which is discovered in great blocks, has so foliated a texture, that the workman needs not exert any attention to catch the joints of its laminæ; wherever he places his chissel, he is sure of hitting a division. A quality common to all clay, is that of furnishing subterraneous reservoirs to fountains and springs; the best water is always found in argillaceous soils. Fermenting clay forms an exception; as it is mixed with Fermentquartz sand reduced to fine powder, it imbibes water, swells ing clay. up, and raises with great force houses, masses of rock, and whole fields; when the clay is dried, they sink down to their first level. The consequences of these unlooked-for throbs and heavings of the soil are very formidable in Sweden, and in Russia, especially when the clay in swelling happens to freeze at the same time. Such devastations, arising from Humboldt, Tableaux de la Nature, ii. 191. Georgi, Description de la Russie, iii. 202, sqq. + Waller, Minéralogie, i. p. 34. Georgi, Russie, iii. 201. BOOK XI, Marl. (Grès.) a cause apparently so weak, will assist us in conceiving of the revolutions which similar fermentations probably accomplished at the epoch when mountains were formed. Marl is a clay united to a greater or less quantity of calcareous earth, often with quartzy sand, and sometimes so impregnated with bitumen, as to take fire of itself.* Siliceous substances are also deposited in beds. SandSandstone. stonet is composed of small grains of quartz agglutinated by a clayey, calcareous, or siliceous cement. It is the most common of all the stratified rocks. It forms the ordinary transition between the primitive and secondary mountains; in this situation it is found in parallel strata, and is but little mixed with heterogeneous substances. But it is met with also at a distance from the primitive mountains, and seems to have been formed at all the geological periods. Recompos There is, as appears to us, a very sensible gradation in the different formations of "grès" or sandstone, from the glossy kind, which nearly approaches the primitive quartz, to the pulviscular species, in which the granulated structure becomes perceptible only by exposure to heat. The recomposed granite, or the sandstone (grès) of the coal ed granite. mines, is an aggregate of minute fragments of the ancient granite rock, united by cement of any kind, and sometimes so exactly resembling the texture of ancient granite, as to deceive naturalists themselves. But as the beds of these secondary granites alternate with those of coal, it is evident that they have a much more recent origin than the rocks of whose wrecks they are composed. We are tempted to notice at this epoch, the enormous blocks of friable granite with which the marshes of Finland are covered, and from among which they have chosen the rock that serves as the base to the statue of Peter the Rapakivi. Great. This granite, named rapakivi, in the language of * Guldenstedt, Voyage dans le Caucase. Wherever the word "grès" is met with, it is best translated by the word sandstone."—T. |