Page:Popular Science Monthly Volume 50.djvu/258

244 have extended through what we term the earth's crust is unknown, but the fact that fissures have in numerous instances become filled with fused rocks is evidence that the breaks referred to are sometimes of sufficient depth to reach regions of intense heat, and many facts seem to favor the idea that this highly heated region is the potentially plastic interior of the earth.

The principle that areas which become weighted by the shifting of material on the earth's surface subside, while unloaded areas rise, has been advocated by several American geologists, and is now common property. It has frequently been asked, however, why an area that is being unloaded should cease to rise so long as erosion continues, and becomes stable or even undergoes a reverse movement; and why an area that is having material added to it should cease to sink and be re-elevated.

In reference to the first of these questions the nature of igneous intrusions seems to furnish an answer. If the rise of a region of denudation is due to an injection of plastic material beneath it, in response to the resulting relief of pressure, the reservoir of highly heated rock forced from below into the cooler rocks above or a protuberance on the surface of the inner sphere will cool, and consequently become more and more rigid. When the intruded rock becomes solid, the portion of the earth's crust lightened by erosion will be increased in weight by material added from below. For the reason that the intruded magma tends to fuse the rocks with which it comes in contact, it will be welded to them as cooling progresses, and when solidification occurs the crust may have greater strength than before the intrusion. Each of these changes would tend to check the upward movement. Dikes and other forms of injections thus tend to strengthen the rocks into which they are forced in much the same way that fractures in the earth's crust are healed and the strata strengthened by the deposition of quartz and other minerals in them so as to form veins. In the case of subterranean injections the cooling of the reservoir of molten rock will be accompanied by contraction, which would cause a subsidence of the surface.

Stating the ideas that I have just attempted to convey more briefly, we should expect that the rise of a denuded area from the effect of internal pressure causing an influx of plastic material beneath it would be checked by the cooling and hardening of the injected material, and a reverse movement or subsidence of the surface initiated as the injected material contracted on cooling.

I am well aware that in these suggestions we are dealing with a single portion of a complex machine and that other results than those considered may follow. That the sole cause of the rise of an eroded area is not the injection of a molten magma beneath is apparent from the fact that such an injection would cause a rise