When it has plenty of room wherein to crystallise, and is not hemmed in, as it were, by other hard crystalline matter, quartz often forms beautiful six-sided prisms surmounted by a six-sided pyramid, and, rarely, pyramids are found at both ends of a prism. There are no lines, or “planes of cleavage,” to interfere with the transparency, either in the extremely minute forms of the mineral as investigated by the microscope, or in the gigantic crystals occasionally found. Regular crystals of quartz, although by no means rare in Nature, are seldom met with entire in brick-earths. The most common form of the mineral is in irregular aggregates with other minerals, as in the rock granite, which is composed essentially, as previously mentioned, of quartz, felspar, and mica. We have traced the history of the felspar on the decomposition of that rock, and it may now be said that on complete disintegration of the granite a great part of the quartz present is simply resolved into fragments and dealt with by rain and other transporting agents. For quartz is practically imperishable; it is almost proof against the deleterious acids in the atmosphere, which so readily attack many other common minerals. In dealing with it, all Nature can do (at least at the surface of the earth) is to carry the small quartz grains and pieces about from place to place; She can, and does, in this process, reduce the quartzose fragments by causing them to continually knock against each other and against other mineral fragments and masses until the grains and pieces find a resting place; She may put them in a mill and grind them to powder, but the quartz is still there.