Page:South African Geology - Schwarz - 1912.djvu/63

 other. If we add enough sugar the water will practically disappear, and a sticky mass will be produced quite unlike the original tea concoction. In rocks, silica takes the place of water, and is called the acid; the other substances, representing the sugar, &c., are called the bases, and consist of alumina, iron, lime, potash, and soda. If there is an excess of silica, the rock crystallizes out as an acid rock; if there is an excess of the bases, then the rock is called a basic rock. If there are equal proportions of acid and bases, the rock is called an intermediate rock.

Another classification is based on the rapidity with which the rock has crystallized. In the depth of the earth the pressure is enormous, and the solution of silica and the bases crystallize out very slowly, so that the crystals are large. These have been called plutonic rocks, because they have formed in the nether regions, over which Pluto is supposed to reign. Offshoots from the main masses of the molten rock are intruded into the surrounding sediments as dykes, and these, being comparatively small bodies, crystallize fairly quickly, therefore the crystals are small. If the dyke finds an outlet to the surface a volcano will be formed, and from the chimney there will eventually issue a stream of molten rock or lava, which will be cooled so quickly that part or the whole of it may not have time to form crystals at all. The following is a diagrammatic scheme of the various rocks. Only those in heavy type will be described, as the characteristics of rocks generally is too big a subject to be considered here: