appears in carbonaceous traces of the stem, branches, or leaves, and sometimes as buried trees. This evidence of its origin gradually vanishes, and the carbonaceous matter assumes the aspect of regularly stratified beds, or layers of an earthy appearance, called pyritous ashes, earth-coal, or marine peat; sometimes it constitutes compact masses of a pretty pure black, of a dense texture susceptible of polish, divided by fissures into rectangular masses, when it is termed brown coal, jet, and even coal. The lignite under these two forms, is either in thin unconnected seams, or in thick beds of great extent; but in both cases, though the ligneous texture should be entirely effaced, some portions of vegetables tolerably well preserved, such as stems, leaves, fruits, are found, which prove that they are derived rather from trees than herbaceous plants, most frequently dicotyledinous, almost always of the family of palms, and never, it is said, of the fern tribe. This circumstance is one of their most remarkable characters, and establishes a very obvious distinction between the ancient and genuine pit-coals, and these deposits of more modern date, to which the name of pit-coal is improperly applied. The fossil shells, which very commonly accompany, and sometimes in prodigious quantity, this upper bed of the plastic clay formation, or the carbonaceous marl, belong to genera, and perhaps species which live in very different elements, some being marine, and others freshwater shelis. These two lie sometimes in thin layers, which touch each other, and though together not a foot thick, yet they are quite distinct. Most commonly, however, SHELLS OF THIS FRESHWATER bed. 319 as in the whole of the Soissonois district, these shells are mixed, but this happens only at the line of contact of the two beds; so that the fresh water shells belong to the lignite stratum, that vast deposit of vegetables which assuredly did not live in the waters of the sea, but grew on the surface of the earth, when that consisted solely of chalk, and was covered with forests, lakes, or marshy pools; whilst the marine shells were bred in the marine deposit, which covers with numerous and massive strata the above argillo-carbonaceous formation. The following list of the fresh water shells has been given by M. de Ferussac-Planorbis rotundatus, incertus, punctum, Prevostinus; physa antiqua; limneus longiscutus; paludina virgula, indistincta, unicolor, Desmaresti, conica, ambigua; melania triticea; melanopsis buccinoidea, costata; nerita globulus, pisiformis, sobrina; cyrena antiqua, tellenoides, cuneiformis. Marine-shells from the intermixture of the superior beds, contain the following: ceritium; ceritium funatum, melanoides; ampullaria depressa; ostrea bellovaca, incerta. The two mineral formations now described must have been produced under entirely different circumstances, since they differ in their chemical nature, in their mode of stratification, and especially in their fossil inmates; so that there is every reason to conclude that the plastic clay belongs to a formation distinct not only from the chalk, but likewise from the superior coarse limestone; since the organic remains appropriate to it, are land or fresh water shells, while all those of that limestone belong to the sea. 3. Of the coarse limestone formation, (calcaire grossier), and its sea-shell sandstones.-This limestone does not always rest immediately on the plastic clay; but is sometimes separated by a bed of sand, sometimes aggregated into freestone. The calcareous formation itself is composed of alternate beds of coarse limestone more or less hard; of argillaceous marl often in very thin layers; and of calcareous marl. It must not however be supposed that these different courses of masonry are placed fortuitously, and without rule, for they always maintain the same order of superposition, over the whole extent of the vast basin. Several seams may be sometimes wanting or very thin; but what is inferior in one district, never becomes superior in another. This constancy in the order of superposition of the thinnest strata, and over an extent of nearly 80 miles, is certainly one of the most remarkable facts disclosed by the researches of MM. Cuvier and Brogniart. The fossil shells furnish the means of recognising amidst so many calcareous layers, any particular one observed in any district however remote. This criterion has never deceived the eminent French naturalists. It is not however to be supposed that the difference between one bed and another is so remarkable, as that between the chalk and the coarse limestone. But the fossils characteristic of one bed become less numerous in the bed above, and disappear entirely in the others, or are gradually replaced by new fossils which had not till then made their appearance. The first and lowest beds of the calcareous formation are the best characterised; they are very sandy, sometimes more so than they are calcareous. When solid, they are apt to decompose in the air and fall to powder; so that the stone can be REMARKABLE SHELL BEDS AT GRIGNON. 321 employed only in very peculiar circumstances. This stratum contains almost always some green earth in powder or grains; which by the analysis of M. Berthier appears to be a silicate of iron. Here is found also a prodigious quantity of fossil shells, the greater part of which differ more from our living species, than the shells of the superior strata do. They are entire, well preserved; and may be easily detached from their rock, retaining in many specimens their pearly lustre. The middle beds also include a great many species of shells; among which are brown impressions of leaves and stems of plants, mixed with cerites, thick ampullaires and other sea-shells. The plants cannot be referred to any marine vegetables. The third or superior system of strata includes fewer shells than the two preceding. The lower seams of it are hard, and are used in building. Above the last beds of the coarse limestone, hard calcareous marls appear, divisible into blocks with their surfaces covered with a yellow varnish, and black dendritic (tree-like) impressions. It appears, therefore, 1. that the fossils of the coarse limestone have been slowly deposited in a tranquil sea, and now lie in regular beds; that they are not mixed; and that the greater portion are in a state of perfect preservation, however delicate their texture may be, since even the points of the spinous shells are very often entire; 2, that these fossils are entirely different from those of the chalk ; 3. that in proportion as the beds of this formation were deposited, the species changed, and several of them disappeared, while new ones made their appear 2 в ance, circumstances which indicate a great many generations of marine animals; lastly, that the number of species of shells always went on diminishing in that region, till the period when they entirely vanished. The waters which deposited these beds, either contained no longer any shellfish, or lost the faculty of preserving them. Certainly things happened in those primeval seas far otherwise from what occurs in our present ocean. In the latter, solid shell-limestone rocks now rise only in tropical climates. The species of shells are not found to change on the same banks and coasts. Ever since oysters have been fished on the coast of Cancale; and mother of pearl aviculæ in the Persian Gulf, it has not been observed that one order of shells has given place to another. 4. Siliceous limestone.-This formation consists of distinct courses, of a limestone sometimes tender and white, sometimes gray and compact, having a very fine grain, and penetrated with silica, filtered through on every point. It is frequently cavernous. These cavities are often pretty large, irregular, and communicate in every direction; they are occasionally cylindrical, but sinuous (winding), and though still irregular, they preserve with one another an appearance of parallelism. The silex by filtering into these cavities, has studded their sides with mammelated stalactites of various hues, or with pyramidal quartz crystals, short and almost without a prism, but distinct and pellucid. This disposition is very |