Evolution of the Earth. 93 



vegetation. If the work of the coral polyps should con- 

 tinue, undisturbed by counteracting agencies, these little 

 (ireatures might iiltimately build a continent five or six 

 thousand miles long, and twenty-five hundred miles in width 

 — a continent larger than Korth America. It has been 

 shown, however, by Professors Le Conte and Dana, that that 

 portion of the earth's crust is subsiding so rapidly that no 

 such result can be reasonably anticipated.* 



The Formation of Igneous Eocks. We have already 

 spoken of the original rock on which the present diversi- 

 fied structure of the earth's surface was founded, as being 

 of igneous origin. It is probable that none of this prim- 

 itive formation is now anywhere visible. Yet everywhere, 

 underneath the stratified rocks, lies a formation of gneiss 

 or granite, or other form of igneous rock. These are all, 

 doubtless, what are termed metamorphic rocks, which, what- 

 ever their original character have been structurally changed 

 and dislocated by the action of heat and the chemis- 

 try of nature. The granite and crystalline rocks, such 

 as we find dominant in the White Mountain system, 

 and to some extent in the Green Mountains and else- 

 where in the Northern part of America, have been 

 formed by igneous agencies, under the immense pres- 

 sure of superimposed masses of rock. By lateral pres- 

 sure, due to the gradual contraction of the earth's 

 crust, and by the slow movements of depression and eleva- 

 tion which are constantly operating, cracks or '* faults " 

 have been formed in the stratified rocks, through which the 

 granite and igneous matter beneath has been pushed up and 

 extruded while in a molten state. Earthquakes sometimes re- 

 sult from this <' faulting " or disarrangement of the strata. 

 These movements are doubtless due, primarily, to the heat 

 of the earth's interior, and secondarily, to the consequent 

 process of cooling and secular contraction. Volcanoes, also, 

 are constantly throwing out lava, and other igneous pro- 

 ducts, thus to a certain extent modifying the earth's sur- 

 face. This infiuence, at pi'esent, is infinitesimal com])ared 

 with the other agencies which we have noted ; though the 

 number of extinct volcanoes, and other evidences of their 

 former action, indicate that in previous ages of the world's 

 history, it was a much more active modifying force than at 



* See Le Conte's " Geology." 



