IGNEOUS INTRUSIONS AND VOLCANOES. 245 



in temperature in the rocks above, and thus by expanding them, 

 increase their upward tendency. The subsequent cooling of these 

 heated and possibly metamorphosed rocks would also tend to re- 

 newed subsidence of the surface. In other words, subterranean 

 intrusions are accompanied by an abnormal rise of the isogeo- 

 therms and their loss of heat by a return to normal conditions. 



In progressively loaded areas, as has been pointed out by 

 Reade, Shaler, and others, there is a blanketing of the earth's heat 

 and a rise of the isogeotherms ; the accompanying expansion of 

 the rocks tends to check subsidence and limit accumulation. The 

 processes of erosion, transportation, and sedimentation in special 

 areas are thus limited by conditions within the earth. Erosion 

 favors elevation until the plastic material transferred to the region 

 beneath the lightened area cools and hardens. A decrease in ele- 

 vation due to contraction then ensues, and is accompanied by a 

 decrease in erosion, which comes to an end when the land is re- 

 duced to base level. Loading favors sedimentation by causing 

 subsidence until the thickened sediments become heated and by 

 reason of their expansion elevate the surface to or above base 

 level. There is a mutual interaction beneath the earth's crust 

 also, since, if only one region of erosion and one of sedimentation 

 are considered, the checking of elevation in a region of erosion by 

 the cooling and hardening of injected magma beneath will give 

 greater resistance to the flow of i3lastic material from beneath the 

 region of sedimentation. 



Deep erosion of subtuberant mountains should reveal a central 

 area of igneous rock surrounded by a belt of metamorphosed 

 rocks which on its outer border, in case the injection occurred in 

 ordinary sedimentary strata, should pass into unaltered sand- 

 stone, shale, etc. In such an instance a radial section should re- 

 veal a gradation from igneous rock through metamorphosed rocks 

 to unaltered sedimentary beds. The breadth of the central core 

 of igneous rock would vary with the size of the intrusion, and, 

 down to a certain limit at least, with the depth of the plane of 

 erosion. One or more generations of dikes might occur in either 

 the central area or in metamorphosed or sedimentary rocks sur- 

 rounding it. Great intrusions if deeply eroded would thus pre- 

 sent the conditions sometimes cited as examples of "regional 

 metamorphism." Some of the features observed in the crystal- 

 line region of Canada seem to illustrate the surface features that 

 would be found if a great subtuberant uplift should be pared 

 away by erosion. 



The fact that a large majority of volcanoes are situated near 

 the sea has led to the supposition that sea water gaining access to 

 highly heated rocks is the chief if not the essential cause of vol- 

 canic eruptions. The hypothesis, however, that the sea is the 



