Crosby.] 482 [November 7, 



subsided to a depth of eight miles ; and over the central portions 

 the principal sediments were impalpable clays and limestones 

 which might have been formed, so far as we can judge from their 

 lithology and paleontology, in a sea two or three miles deep. 

 This Paleozoic sea covered the major part of the continent till 

 the Carboniferous age — a period estimated by geologists at from 

 ten million to fifty million years, or three times as long as the 

 Mesozoic and Cenozoic eras combined. It is not easy to see how 

 North America, during these millions of years, was any more 

 continental than Oceania is now. 



One of the most important questions in American geology is 

 that as to the source of the Paleozoic sediments in the Appala- 

 chian region. The volume of these sediments is enormous, meas- 

 uring 30,000 to 40,000 feet in thickness for a breadth of probably 

 100 miles and a length several limes greater. The Paleozoic 

 shore line lay to the southeast and all this mass of material evi- 

 dently came from that direction. But the only pre-Paleozoic 

 land now visible between New York and North Carolina is a 

 narrow belt of crystalline rocks varying in width from nothing to 

 sixty or eighty miles. This has been broadened by the erosion of 

 the Paleozoic sediments ; though narrowed somewhat by the de- 

 position upon it of the Mesozoic and Tertiary beds on the east. 

 If it never extended beyond the present shore line, it certainly 

 cannot be regarded as an adequate source of the vast piles of 

 Appalachian sediments. Some geologists, appreciating this dif- 

 ficulty, have supposed that this narrow belt of Paleozoic land was 

 renewed by elevation as fast as destroyed by erosion, the elevation 

 required being not less than the thickness of the derived sedi- 

 ments, or eight miles. But eight miles of subsidence on one side 

 of the shore line and eight miles of elevation on the other side 

 implies a pretty flexible crust ; and the advocacy of this violent 

 hypothesis by eminent geologists shows how far some of the be- 

 lievers in stable continents are willing to go rather than disturb 

 the ocean floor ; although, according to this theory, the oceanic 

 crust is newer and more flexible than the continental. Most Amer- 

 ican geologists, however, including Professor Dana, have solved 

 this problem of the Appalachian sediments by broadening the belt 

 of Paleozoic land. Professor Dana conceives that it extended 

 as far east as the existing shore line, perhaps beyond it (although 



