182 A CENTUBY OF SCIENCE 



offer an argument of great weight respecting the earth 's interior 

 condition, and make it desirable that the subject should be dis- 

 cussed in this connection. Moreover, the facts throw additional 

 light on the preceding topic the origin of mountains. 



It seems now to be demonstrated by astronomical and physical 

 arguments arguments that are independent, it should be noted, 

 of direct geological observation that the interior of our globe 

 is essentially solid. But the great oscillations of the earth's 

 surface, which have seemed to demand for explanation a liquid 

 interior, still remain facts, and present apparently a greater 

 difficulty than ever to the geologist. Professor Le Conte's views, 

 in volume iv, were offered by him as a method of meeting this 

 difficulty; yet, as he admits in his concluding remarks, the 

 oscillations over the interior of a continent, and the fact of the 

 greater movements on the borders of the larger ocean, were 

 left by him unexplained. Yet these oscillations are not more 

 real than the changes of level or greater oscillations which 

 occurred along the sea border, where mountains were the final 

 result; and this being a demonstrated truth, no less than the 

 general solidity of the earth's interior, the question comes up, 

 how are the two truths compatible? 



The geological argument on the subject (the only one within 

 our present purpose) has often been presented. But it derives 

 new force and gives clearer revelations when the facts are viewed 

 in the light of the principles that have been explained in the 

 preceding part of this memoir. 



The Appalachian subsidence in the Alleghany region of 35,000 

 to 40,000 feet, going on through all the Paleozoic era, was due, 

 as has been shown, to an actual sinking of the earth's crust 

 through lateral pressure, and not to local contraction in the 

 strata themselves or the terranes underneath. But such a sub- 

 sidence is not possible, unless seven miles that is, seven miles 

 in maximum depth and over a hundred in total breadth unless 

 seven miles of something were removed, in its progress, from 

 the region beneath. 



If the matter beneath was not aerial, then liquid or viscous 

 rock was pushed aside. This being a fact, it would follow that 

 there existed, underneath a crust of unascertained thickness, a 

 sea or lake of mobile (viscous or plastic) rock, as large as the 

 sinking region; and also that this great viscous sea continued 

 in existence through the whole period of subsidence, or, in the 

 case of the Alleghany region, through all Paleozoic time an era 

 estimated on a previous page to cover at least thirty-five millions 

 of years, if time since the Silurian age began embraced fifty 

 millions of years. 



The facts thus sustain the statement that lateral pressure 



