Geology is the study of the rocks or the substances of which the earthy crust of a district is composed. Rocks are of two sorts: (1) those due to the action of heat, called igneous, (2) those formed and deposited by water, called aqueous. When the earth was a molten ball, it cooled at the surface, but every now and again liquid portions were ejected from cracks and weak places. The same process is seen in the eruptions of Mount Vesuvius, which sends out streams of liquid lava that gradually cools and forms hard rock. Such are _igneous_ rocks. But all the forces of nature are constantly at work disintegrating the solid land; frost, rain, the action of rivers and the atmosphere wear down the rocks; and the tiny particles are carried during floods to the sea, where they are deposited as mud or sand-beds laid flat one on the top of the other like sheets of paper. These are _aqueous_ rocks. The layers are afterwards apt to be tilted up on end or at various angles owing to the contortions of the earth’s crust, through pressure in particular directions. When so tilted they may rise above water and immediately the same process that made them now begins to unmake them. They too may in time be so worn away that only fragments of them are left whereby we may interpret their history. [Illustration: Loch of Skene] To these may be added a third kind of rock called _metamorphic_, or rocks so altered by the heat and pressure of other rocks intruding upon them, that they lose their original character and become metamorphosed. They may be either sedimentary, laid down originally by water, or they may be igneous, but in both cases they are entirely changed or modified in appearance and structure by the treatment they have suffered.