Proceeding with our rapid sketch of the crust of the earth and its uses, we now leave the Lias, to enter upon a survey of the Oolite proper. Overlying the Lias, and underlying the Wealden, we find this deposit, which, though it occupies a comparatively narrow track in our own country, is remarkable for the peculiarity and beauty of its fossils, and for the commercial importance of the rocks of which it is composed. Its name is derived from _ōŏn_, an egg, and _lithos_, a stone,[88] from the remarkable resemblance many of the beds bear to the roe, or eggs of a fish. A good specimen of oolite and the hard roe of a red herring are unlike one another, mostly in the circumstance that one can be cooked, and the other cannot. These egg-like grains are mere agglomerations of calcareous matter, although sometimes a piece of coral, or a broken shell, or a grain of sand, is found to be the nucleus around which these deposits have arranged themselves. It may be as well here to say a word upon the general character of _calcareous_ rocks, which are so largely to engage our attention in this and a subsequent chapter. “This division comprehends those rocks which, like chalk, are composed chiefly of lime and carbonic acid. Shells and corals are also formed of the same elements, with the addition of animal matter. To obtain pure lime it is necessary to calcine these calcareous substances,—that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid and other volatile matter, without vitrifying or melting the lime itself. White chalk is often pure carbonate of lime; and this rock, although usually in a soft and earthy state, is sometimes sufficiently solid to be used for building, and even passes into a compact stone, or a stone of which the separate parts are so minute as not to be distinguishable from each other by the naked eye. “Many limestones are made up entirely of minute fragments of shells and corals, or of calcareous sand cemented together. These last might be called ‘calcareous sandstones;’ but that term is more properly applied to a rock in which the grains are partly calcareous and partly siliceous, or to quartz-ore sandstones having a cement of carbonate of lime. “The variety of limestones called ‘_oolite_’ is composed of numerous small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated.