Crosby.] 484 [November 7, 



particular, to the subsidence of the sea bottom. But the conti- 

 nents are the portions of the crust lifted above the general level 

 and held in unstable equilibrium, and their tendency to subside 

 is so great as, according to Professor Le Conte, to be almost irre- 

 sistible. While it would seem equally natural and necessary for 

 the ocean bed to rise when the strain is relieved which holds it 

 down. 



Professor Dana regards the floor of the great interior Paleozoic 

 sea between the Blue Ridge and the Rocky Mountains as having 

 been essentially a part of the continent during all the millions of 

 years when it was submerged, and not as having been the bottom 

 of a true ocean. Therefore he could not attribute the Allegha- 

 nies to its subsidence. Besides, this sea-floor rose when the Alle- 

 ghanies were made, and all geologists agree that the character of 

 the Alleghany foldings shows conclusively that the pressure pro- 

 ducing them came chiefly from the southeast. Hence, Professor 

 Dana, believing that the Atlantic existed with nearly its present 

 outlines during Paleozoic time, says that the Alleghanies were 

 formed by the subsidence of the great arch of the Atlantic. But, 

 as Ave have seen, he admits that the Appalachian sediments were 

 separated from the Atlantic by at least one hundred and possibly 

 several hundred miles of firm land. In other words, Professor 

 Dana tells us that the sediments deposited in one ocean were 

 plicated by the subsidence of the floor of another and entirely 

 distinct ocean. This is very much the same as saying that the 

 sediments now accumulating in the Gulf of Mexico will some day 

 be compressed and folded by the subsidence of the bottom of the 

 Pacific, Central America and Mexico remaining undisturbed as at 

 present. 



The Triassic and other Secondary deposits upon our Atlantic 

 sea-board make it impossible to doubt that a very general and 

 extensive subsidence of the land in this quarter did take place at 

 the time of the Alleghany revolution. Here is positive proof of 

 the subsidence of a very extensive land area, and of land, too, 

 immediately adjoining the Alleghany sediments - but where is 

 there a vestige of reliable evidence showing that the floor of the 

 Atlantic subsided at this epoch, or even that the Atlantic was 

 then in existence ? 



The Atlantic continent helps us over many difficult points in 



