Limestone.—This is the lithologic or rock form of carbonate of lime or calcite, and one of the most important, interesting, and useful of all rocks. Although so simple in composition,—calcite being the only essential constituent,—limestone embraces many distinct varieties, and is really equivalent to a whole family of rocks. A highly fossiliferous limestone, such as specimen 38, is, perhaps, the best variety with which to begin the study of the species. The softness of the fossil shells of which the rock is so largely composed, and the fact that they effervesce readily with dilute acid, proves that they are still carbonate of lime; and by applying the acid more carefully, it can be seen that the compact matrix of the rock also effervesces, consisting of shells more finely broken or comminuted and mixed with more or less clay and other impurity, almost the entire rock being of organic origin. On the coast of Florida, and in many other places, we find beautiful examples of shell-limestone now in process of formation. These are at first very open and porous, because the interstices between the nearly entire shells are not yet filled up with smaller fragments and sand. But when that is done, we shall have a rock similar to the old fossiliferous limestone. Specimen 37. The shells and fragments, and the grains of calcareous sand, are, as a rule, quickly cemented together by the deposition of carbonate of lime between them; so that limestone is nowhere observed occurring abundantly in an unconsolidated form. Limestone, as a rule, is not distinctly stratified in hand-specimens, but of course that it is a true sedimentary rock is abundantly proved by the fossils; and it goes almost without saying that limestone, being necessarily mainly composed of organic remains, must be to a greater extent than any other rock the great store-house of fossils; and in no other rock are the fossils so well preserved and perfect as in limestone. Nevertheless, there are extensive formations of limestone containing no discernible traces of fossils. And some of these non-fossiliferous limestones, too, are of very recent formation. Some of the modern coral-reefs, for example, are composed of limestone which was formed only yesterday, as it were, and which, from its mode of formation, must consist entirely of corals; and yet it shows no trace of its organic origin, but is perfectly compact, or, possibly, crystalline. This frequent obliteration of the organic remains, as well as the perfect consolidation of the rock, is attributed to its solubility. The calcium carbonate is gradually dissolved by the water, and then deposited in the interstices in other parts of the rock.