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.
Specimen 39 is that variety of limestone known as _chalk_. It is soft and earthy, resembling both clay and tripolite, but differing from the former in lacking the distinct argillaceous odor, and from both by its lively effervescence with acids. It appears to be entirely destitute of organic remains, but this is a defect of our vision and not of the rock; for, like the tripolite, it often appears under the microscope to be little else than a mass of shells. Tripolite is a deposit built up of the siliceous shells of Diatoms and Radiolaria, while chalk is chiefly composed of the similar but calcareous shells of Foraminifera. Our specimen is from the Cretaceous formation of England; but we have good reason to believe that chalk is _now forming_ on a very extensive scale. There are millions of square miles in the deeper parts of the ocean where the dredge brings up little else but a perfectly impalpable, gray, calcareous slime or ooze. When examined microscopically, this is seen to be composed chiefly of Foraminifera shells, and among these the genus Globigerina predominates; so that the deposit is frequently called Globigerina ooze. Now this gray, calcareous ooze, when dried and compacted by pressure, makes a soft, _white_ rock which can scarcely be distinguished from chalk; in fact, it is a modern chalk. And there seems no good reason to doubt that the deposition of chalk has gone on continuously since Cretaceous time—for several millions of years at least.