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. Specimen 40 is also a white rock, easily scratched with the knife, and effervescing freely with acid, and therefore a variety of limestone. But its texture is very different from the other varieties we have studied. It has a sparkling surface, which we explain by saying that the rock is crystalline. It is, in fact, a mass of minute crystals of calcite. The crystalline limestones have not always been crystalline, but it is safe to assume that they were originally entirely uncrystalline, and in many cases rich in fossils; but the fossils have been mainly obliterated by the crystallization. Crystallization generally in rocks is an indication of great age, so that we usually say crystalline rocks must be older than uncrystalline rocks of the same composition; and this is mainly true with the limestones. When the crystallization is rather fine, as in our specimen, resembling granulated sugar, we have what is commonly called saccharoidal limestone. This is the typical marble. Marble is not a scientific name, and the term is usually applied to any calcareous rock which will take a polish, and sometimes even to rocks which are not calcareous at all.