Another manner in which quartz occurs in Nature is as filling cracks in rocks, but this is comparatively unimportant for our present purposes. The purest quartz is known as rock crystal; but by far the commonest kinds of the mineral are impure; they may contain iron, schorl (a black needle-like crystal), and many other minerals. One of the most interesting points about it, and which undoubtedly in certain cases is of importance to the brick manufacturer as modifying its melting properties, is the presence of myriads of extremely minute so-called cavities, generally filled (or nearly filled) by liquids of different kinds, the precise nature of which is not as well-known as it might be, though in some instances it has been determined with tolerable certainty. In some cases these inclusions are so numerous as to obliterate the transparency of the quartz crystal, causing it to present a frosted appearance. The fluids in these cavities may have beautiful little crystals of other minerals, such as salt, floating about--but it must be remembered that we are referring to something infinitely little. These slight differences in the constitution of minerals, however, have their influence in the kiln. For instance, although the fluid present is usually water, that often contains carbon dioxide, which acts as a species of flux to the quartz when present in sufficient quantity. In reference to the second form of silica present in brick-earths, flint, that is of precisely the same chemical composition as quartz, only that it is not crystalline, nor transparent, though thin pieces of flint are translucent. Flint is by no means as common in Nature as quartz; it is very hard, but brittle, and breaks with what is termed a conchoidal fracture, from the fact that the fractured surface frequently resembles the external appearance of the shell of a bivalve mollusc. It occurs in a variety of ways; (1) often as hard lumps or nodules running along in fairly regular layers in limestone rocks such as chalk, and (2) occasionally filling up cracks or joints in such rocks. It is hard to describe its origin in a few words, and we shall not attempt it; all that need be noted is that it is frequently full of the remains of extinct organisms of small size, which may, or may not, constitute an impurity depending on the particular organism and its present condition. When flint contains a fair proportion of iron it is called chert--an extremely common constituent of brick-earths in some localities--though that term refers to other rocks, such for instance, as those made up almost exclusively of the siliceous spicules (hard parts made of silica) of fossil sponges. A more or less crystalline kind of silica is found, forming the skeletons of minute aquatic plants, and these accumulating to some depth, constitute the basis of such materials as Kieselguhr and the diatom earth of the Isle of Skye, both of which, especially the former, are used for making firebricks.