1883.] 465 [Crosby. 



are merely local phenomena. On the contrary, all will concede 

 that it is more reasonable to suppose that the area affected is, on 

 the average, roughly proportional to the change of level. Now 

 the subsidence of 40,000 feet in the Alleghany region during Pa- 

 leozoic time did not make a deep ocean there, because deposition 

 kept pace with the downward movement. But where is the evi- 

 dence that the subsidence was limited to the eastern border of 

 the great Paleozoic sea ? Are we not, in accordance with the 

 foregoing, at liberty to conclude that it affected, perhaps in equal 

 measure, the central portions of the sea, where the deposition was 

 only one-tenth as rapid as in the east ? To answer in the affirm- 

 ative is to admit that a large part of the present continent be- 

 came the site of the abyssal ocean. If, as we believe, the great 

 oscillations of the crust go on independently of deposition, it is 

 certainly strange that they should be limited to the neighbor- 

 hood of coast lines. Professor Dana admits that the important 

 upward movements of the crust affect extensive areas, as witness 

 the elevation of North America, Europe, and Asia in the Tertiary 

 period, and the elevation of South America at the present time ; 

 and no good reason is apparent for denying that the same holds 

 true with important downward movements. The evidence of 

 elevation is, from the nature of the case, abundant and positive ; 

 while the evidence of subsidence is meagre and negative ; except 

 along the coast-lines where deposition measures the movement. 

 But speculation is unnecessary here, because the coral-islands of 

 the Pacific are monuments of a subsidence which is at once pro- 

 found and wide-spread. 



Professor Dana agrees with most American geologists that 

 during Paleozoic time a considerable body of land lay to the east- 

 ward of the present Atlantic coast-line of the United States. 

 But, if it extended very far in that direction — more than one 

 hundred miles — it must be now in part under one to two miles 

 of water. According to Professor Dana and the great majority 

 of geologists, the important movements are necessarily reciprocal, 

 one part of the crust rising as another part sinks ; and Professor 

 Dana says farther that the oceanic crust is more flexible and rests 

 on more mobile material than the continental. Why, then, 

 should he, with the certain knowledge of a Paleozoic subsidence 

 of 40,000 feet in the Alleghany region, a Mesozoic subsidence of 



PROCEEDINGS B. S. N. H. VOL. XXII. 30 AUGUST, 1884. 



