2. The calcareous strata are the portion of the mineral kingdom that gives the clearest testimony to the truth of this assertion. They often contain shells, corals, and other exuviæ of marine animals in so great abundance, that they appear to be composed of no other materials. Though these remains of organized bodies are now converted into stone or into spar, their shape and interior structure are often so well preserved, that the species of animal or plant of which they once made a part, can still be distinguished and pointed out among the living inhabitants of the ocean. Others of the calcareous strata appear to be composed of fragments of some ancient rocks, which, after having been broken, have been again united into a compact stone. In these we find pieces clearly marked as having been once continuous but now placed at a distance from one another, and exhibiting exactly the same appearances as if they floated in a fluid of the same specific gravity with themselves. From these, therefore, and a variety of similar appearances, Dr Hutton concludes, that the materials of all the calcareous strata have been furnished, either from the dissolution of former strata, or from the remains of organized bodies. But, though this conclusion is meant to be extended to all the calcareous strata, it is not asserted that every cubic inch of marble or of limestone contains in it the characters of its former condition, and of the changes through which it has passed. It may, however, be safely affirmed, that there is scarce any entire stratum where such characters are not to be found. These must be held as decisive with respect to the whole system of strata to which they belong; they prove the existence of calcareous rocks before the formation of the present; and, as the destruction of those is evidently adequate to the supply of the materials of these that we now see, to look for any other supply were superfluous, and could only embarrass our reasonings by the introduction of unnecessary hypotheses.[2] [Footnote 2: NOTE I.] 3. The same conclusions result from an examination of the siliceous strata; under which we may comprehend the common sandstone, and also those pudding-stones or breccias where the gravel consists of quartz. In all these instances, it is plain, that the sand or gravel existed in a state quite loose and unconnected, at the bottom of the sea, previous to its consolidation into stone. But such bodies of gravel or sand could only be formed from the attrition of large masses of quartz, or from the dissolution of such sandstone strata as exist at present; for it will hardly be alleged, that sand is a crystallization of quartz, formed from that substance, when it passes from a fluid to a solid state.