INTERIOR OF THE LITHOSPHERE. Early geol ogists considered the interior of the earth to be molten, basing their conclusion upon a number of facts pointing to a high temperature for the interior. The numerous hot springs indicate heated conditions below the surface; all deep borings and mines show a rise in the temperature with increasing depth; and volcanoes actually bring melted rock to the surface. The move ments of the crust also may be accounted for by assuming a heated interior, which upon cool ing and shrinking allows the -cold, solid crust to settle on it and wrinkle. If the observed in crease in temperature in mines and borings, which averages one degree for every 50 to 60 feet of descent, is continued far into the earth, tem peratures must eventually be encountered which are above the melting-point of rocks at the sur face.
Astronomers and terrestrial physicists have shown, however, that the earth cannot be a mol ten sphere with a thin crust. In its behavior toward other members of the solar system the earth acts like a solid body, and one as rigid as steel. If there is a solid crust it must be at least 2500 miles thick. The evidences for this conclusion are obtained not only from the be havior of the earth toward other members of the solar system, but also from the absence of tides which would be present in a molten interior, and from the fact that the average density of the earth is far greater than that of the rocks at the surface, indicating a very dense, heavy interior.
Geological facts also point toward the conclu• sion that the earth's interior is not molten. Con sequently geologists have long accepted the hy pothesis of a solid heated interior, so hot that it would be molten under normal conditions, but kept from melting by the enormous load of the crust, since the melting-point of rocks is raised with an increase in pressure. Whether there is a zone of molten rock between the solid cold crust and the solid heated interior is not known. Many believe that the rock of the interior is molten only where the pressure is relieved by the uparching of the crust under mountain The condition in which the heated rock exists in the interior is one of the fundamental problems of geology still awaiting solution.