It is easy to understand why shells should prevail chiefly in the calcareous strata, because it is very apparent that the larger portions, probably, in some cases, the whole of these, have been produced from the destruction of shells. It is almost equally easy to understand why, of the other strata, they abound more in the argillaceous than in the siliceous ; in the shales rather than in the sandstones. Such a substance as clay we know to be more congenial to the living animals, to many of them, at least, than sand ; and it is thus by attending to the various habits and pro pensities of different kinds of these, that we can explain a number of minute circumstances respecting their petrified or preserved state, on which it would be here superfluous to dwell. That they now reside in tribes or colonies, that they arc partially distributed through the sea, and that they are occasionally exterminated from some particular tract where they once inhabited, are facts in their living history, easily applicable to the explanation of all the peculiarities which they present when involved in stone, and on which we might, were it necessary, dwell at great length. In many places, they have been broken to pieces ; in others, it is probable, they have been so comminuted as entirely to lose their forms, producing mud and subsequent simple limestones in some instances, while, in others, having been ground into the state of sand by the action of the sea, they have generated oolites. This very process is indeed visi ble in the accumulations of calcareous sand so abundant in many parts of the West Indies, among which we may even trace the very progress of induration into stone. The same happens in the coral islands of the Pacific Ocean ; since the key, or outer and windward bank of all these ex traordinary productions, consists of stratified limestones formed out of the reunited fragments of the corals which have been broken off by the violence of the sea, and after wards indurated by means of its solvent and consolidating power.
There are yet some peculiar cases, where limestones do not contain organic remains where they might be expect ed; where, for example, the general mass, or body of strata, may contain them, while other parts are exempt. Re markable instances of this nature are described by Dr. Macculloch, in Sky, and in the Isle of Man ; and they are shown to have arisen from posterior changes in the chemi cal constitution of the rock in those particular spots. These changes consist in a crystalline texture of the rock in those places, evidently induced by the partial fusion of the strata from the agency of trap rocks ; just as happens in those cases where chalk, under the same circumstances, is converted into crystalline limestone. In this state of fusion the organic substances have themselves been melted and confounded with the general mass ; nor, indeed, is it difficult, in sume cases, to trace the whole progress of this change ; the organic forms losing their accuracy exactly in proportion to the more perfect crystallization of the lime stone.
It is chiefly, therefore, in the limestones and shales among the solid strata, and, of course, in the secondary rocks, that we may expect to find animal remains. But they occur, also, in the sandstones that accompany these, although in far less abundance and variety. They are also found imbedded in the unconsolidated strata that alternate with these, as in the clays, sands, and marls. But above these, also, they are found in those deposits now understood to have been formed under fresh water ; and, in these cases, they occur, further, in gypsum, and in a peculiar siliceous or cherry rock, as well as in the mote ordinary substances already mentioned.
We must now, however, turn our thoughts backwards to the rocks of an older date, in which they do also occur, although far more rarely, and in less variety and abundance at the same time. As we do not use the term transition, nor admit that there is any such class of rocks distinguish able by any assignable characters, it is here proper to re mark, in the first place, that, with many geolagists, we consider all rocks as most conveniently separated into two classes, the primary and secondary. The lowest sandstone is esteemed the first, or lowest of the latter class, and therefore all the strata which succeed to it downwards, are in the primary division.
Now, in this division it is well known that organic bo dies, chiefly marine, occur in the'argillaceous schists, and that, often, in very great abundance. In Cornwall, this occurrence is not unfrequent; and in North Wales it is so common, that whole beds are found whicli almost seem composed entirely of shells. The summit of Snowdon is an example of this fact. With respect to the primary limestone, there is more difficulty ; as, under the term transition, both the primary and secondary strata have so often been confounded, that we cannot trust to the des criptions of authors respecting them. It is, however, na tural to expect, that, if such remains are found in primary argillaceous schists, they are mush more likely to exist in the calcareous strata of the same class, for the reasons al ready assigned as the causes of their frequency in the se condary limestones. These, then, are the only primary rocks in which organic remains have been found so fre quently as to be quite familiar. But one instance is related by Dr. in his description of some rocks in Sutherland, where small orthoceratites exist in a quartz rock containing a mixture of calcareous particles, and in terstratified with other primary rocks, such as gneiss. It is true, that fishes have been found in a substance called bituminous marl slate, known in many parts of Europe, and said to be a primary rock But there is probably some fallacy respecting the real class of this substance ; and in deed it is evident,from the descriptions of some geologists, that this stratum is an occasional or subsidiary member of some of the secondary calcareous groups The last strata to be mentioned, in which organic re mains are found, are the alluvial soils ; and it is in these that nearly all the more perfect animals have been disco vered. Such remains, indeed, have attracted more atten tion than all the others, from their greater conspicuity, and their frequent resemblance to the present inhabitants of the earth. They also bespeak great past revolutions, but of a different nature from those which are indicated by the remains that are inclosed in the solid strata. Such allu vial soils vary in their natures in different places ; and as it is plain that they must generally depend on the nature of the rocks out of which they have been formed, it would be useless to describe them minutely. In texture, they may be fine or coarse, or both intermixed ; and they must con sist of gravel, sand, and clay, the nature of which will vary according to the causes already mentioned, without affect ing the characters of the included bodies.