. Elements of geology : a text-book for colleges and for the general reader. Geology. 86 IGNEOUS density toward the center, we must seek this temperature at a greater depth. If A B (Fig. 76), representing depth from the surface S S, be taken as absciss, aud heat be represented by ordinates, then, in a homogene- ous earth, C D would represent uniform increase of heat, and the heat ordinate of 3,000°, m m, would be reached at the depth of A m = thirty miles. But in an earth increasing in density, and, there- fore, in conductivity, the rate would not be uniform, but gradually decreasin
. Elements of geology : a text-book for colleges and for the general reader. Geology. 86 IGNEOUS density toward the center, we must seek this temperature at a greater depth. If A B (Fig. 76), representing depth from the surface S S, be taken as absciss, aud heat be represented by ordinates, then, in a homogene- ous earth, C D would represent uniform increase of heat, and the heat ordinate of 3,000°, m m, would be reached at the depth of A m = thirty miles. But in an earth increasing in density, and, there- fore, in conductivity, the rate would not be uniform, but gradually decreasing. This would be represented, not by a straight line, C D, but by a curved line, C E; and the ordinate of 3,000° would not be reached at thirty miles, but at a much greater depth—say at m\ of fifty miles. 2. Fusing-Point not the same for all Depths.—Nearly all substances expand in the act of melting, and contract in the act of solidifying. Only in a few substances, like ice, is the re- verse true. Now, the fusing point of all substances which expand in the act of fusing must be raised by pressure, since the expanding force of heat, in this case, must overcome not only the cohesion, but also the pressure. That this is true, has been proved experimentally for many substances by Hopkins.* But granite and other rocks have been proved to expand in fusing; therefore the fusing-point of rocks is raised by pressure, and must be greatly raised by the inconceivable pressure to which they are subjected in the interior of the earth. For this reason, therefore, we must again go deeper to find the interior fluid. In the figure, m! is the point where we last found the fusing-point of 3,000°. But this is the fusing-point on the surface, or under atmos- pheric pressure. The pressure of fifty miles of rock would certainly greatly raise the fusing-point. Suppose it is thus raised to 3,500° : to find this we must go still deeper, to m", perhaps seventy-five miles in depth. But the inc
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