244 POPULAR SCIENCE MONTHLY. 



have extended througli what we term the earth's crust is un- 

 known, but the fact that fissures have in numerous instances be- 

 come 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 vfhich 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 igne- 

 ous 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 depo- 

 sition 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 



