The average distance from the earth's surface to its center is sixty-three hundred and seventy kilometres. The temperature of the earth increases with the depth, at the rate, on a moderate estimate, of about one degree centigrade for every forty metres. Hence, at a depth of one thousand kilometres we would have a temperature of 25,000° C.; even if we call it only 15,000°, we should expect to find there only gases, and those in a simple state, for with that heat all the compound gases would be dissociated. The zone of fluidity for all rocks lies at a depth of about one hundred kilometres, where the temperature is 2,500° C. While the crust of the earth is between 2.5 and three times as heavy as distilled water at 4° C., its specific gravity rises toward the center of the earth to more than eleven, or about fourfold. Iron has a specific gravity of 7.8, or about threefold that of the crust of the earth; but the specific gravity of the earth at the greatest depth is considerably higher than this. Hence must arise an enormous pressure, steadily increasing toward the center, where, according to the English geophysicist, the Rev. Osmond Fisher, it reaches about three million atmospheres to the English square inch. It results from these conditions that with the enormous pressure and heat, and specific gravity, the interior of the earth consists of dissociated gases compressed to great rigidity, which exert an immense counter-pressure--for their tendency is always to expand. They pass out continuously into a zone of fluid matter, and this again is held by the pressure of the interior gases in a like compact condition. Thus a very high pressure still prevails in the lower parts of the solid crust of the earth, which is so high that even the most solid rocks there are in a latent plastic condition--that is, they behave toward different forces like plastic clay, and like it can be deformed without breaking. Rents, slides, caves, and clefts are out of the question there; things of that kind can exist only in the upper strata.