[Illustration: Fig. 11.--Section of Carboniferous Limestone from Spergen Hill, Indiana, U.S., showing numerous large-sized _Foraminifera_ (_Endothyra_) and a few oolitic grains; magnified. (Original.)] [Illustration: Fig 12.--Section of Coniston Limestone (Lower Silurian) from Keisler, Westmoreland; magnified. The matrix is very coarsely crystalline, and the included organic remains are chiefly stems of Crinoids. (Original.)] Many limestones, as we have seen, are composed of large and conspicuous organic remains, such as strike the eye at once. Many others, however, which at first sight appear compact, more or less crystalline, and nearly devoid of traces of life, are found, when properly examined, to be also composed of the remains of various organisms. All the commoner limestones, in fact, from the Lower Silurian period onwards, can be easily proved to be thus _organic_ rocks, if we investigate weathered or polished surfaces with a lens, or, still better, if we cut thin slices of the rock and grind these down till they are transparent. When thus examined, the rock is usually found to be composed of innumerable entire or fragmentary fossils, cemented together by a granular or crystalline matrix of carbonate of lime (figs. 11 and 12). When the matrix is granular, the rock is precisely similar to chalk, except that it is harder and less earthy in texture, whilst the fossils are only occasionally referable to the _Foraminifera_. In other cases, the matrix is more or less crystalline, and when this crystallisation has been carried to a great extent, the original organic nature of the rock may be greatly or completely obscured thereby. Thus, in limestones which have been greatly altered or "metamorphosed" by the combined action of heat and pressure, all traces of organic remains become annihilated, and the rock becomes completely crystalline throughout. This, for example, is the case with the ordinary white "statuary marble," slices of which exhibit under the microscope nothing but an aggregate of beautifully transparent crystals of carbonate of lime, without the smallest traces of fossils. There are also other cases, where the limestone is not necessarily highly crystalline, and where no metamorphic action in the strict sense has taken place, in which, nevertheless, the microscope fails to reveal any evidence that the rock is organic. Such cases are somewhat obscure, and doubtless depend on different causes in different instances; but they do not affect the important generalisation that limestones are fundamentally the product of the operation of living beings. This fact remains certain; and when we consider the vast superficial extent occupied by calcareous deposits, and the enormous collective thickness of these, the mind cannot fail to be impressed with the immensity of the period demanded for the formation of these by the agency of such humble and often microscopic creatures as Corals, Sea-lilies, Foraminifers, and Shell-fish.