Page:Popular Science Monthly Volume 54.djvu/384

368 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.

This fact constitutes a very strong objection to the tectonic theory of earthquakes, and thus the very depths of the earth speak against it. We have already mentioned that K. von Seebach estimated the depth of the earthquake focus from the movements of the waves, and found it not very great. But his estimates, as Prof. August Schmidt has shown, rest upon physically incorrect premises; according to Schmidt's more correct calculation, the center of the Charleston earthquake of 1886 lay at a depth of one hundred and twenty kilometres, where there can be no question of tectonic movements, because general fluidity is reached at one hundred kilometres. Further, the earthquake at Lisbon, if the tectonic theory is valid, might, taking the character of the region into consideration, have been occasioned by a slide. But how large must the plunging mass, how deep the plunge or slide have been to produce such shocks as destroyed Lisbon and shook Europe to beyond Bohemia! Where can we find room in the closely compressed interior of the earth for such irruptions? Even if such a sudden sinking had left no trace in the interior, it should have left its marks on the surface. Mr. John Milne counts up not less than 8,331 considerable earthquake shocks