THE behaviour of materials such as granite and basalt can be tested in the laboratory up to a pressure of 30,000 atmospheres, or, as engineers would say, about 200 tons to the square inch. This corresponds to a depth of 120 km. in the earth's crust, but what are the conditions of matter at pressures of 10,000 tons to the square inch or thereabouts, which presumably must obtain near the centre of the earth? Then with regard to temperature, we know that near the surface of the earth in Europe this increases generally with depth by about 30° C. per km. The rate of increase differs considerably from one point of the earth's surface to another. In North America it is three quarters of this amount, and in South Africa only about one half. In regions where volcanic activity has taken place in recent geological times, the rates of increase are abnormally high. In a boring almost a kilometre deep at Budapest a temperature of 69° C. was reached at the bottom. In Tuscany and other places in Italy a similar rate of increase has been found. If we take an average and extrapolate from this, we find that at a depth of something like 50 km. we reach a temperature of 1,400° C.,—enough to fuse all but the most refractory substances.