1883.] 449 [Crosby. 



have extended east beyond the existing border of the continent. 

 And he further states that "the under-crust fire-sea on the Pacific 

 border must have had great length from northwest to southeast ; 

 and, also, great breadth, for the border region is at least 1,000 

 miles wide ; and great breadth and great length seem plainly its 

 characteristics even till Tertiary times." Again, " It is further to 

 be noted that, in the course of past time, the whole continent has 

 had its surface, from one side to the other, criss-crossed with os- 

 cillations and lines of disturbance, from the lateral pressure act- 

 ing against its opposite sides, whence it is clear that the conti- 

 nental subterranean areas were once continuous. The facts from 

 the ocean seem to demand a vastly greater range for the under- 

 crust mobile layer." And, finally, in conclusion, Professor Dana 

 says there is a flexible crust and mobile rock beneath it. 



Thus we find Professor Dana fully committed to the view that 

 the earth contains a nearly continuous plastic zone; although his 

 theory of continents, as will appear farther on, appears to require 

 that the continents should be for the most part at least, rigidly 

 connected with the earth's solid nucleus. We hardly seem war- 

 ranted, however, in assuming — for it certainly has not been 

 proved — that a nearly if not quite universal mobile layer, which 

 we are compelled by the phenomena of elevation and subsidence 

 to believe was in existence from the period when the entire globe 

 was an incandescent liquid down to the later Tertiary times, sud- 

 denly became solid under the continents with the close of that 

 epoch. 



Besides, we have abundant evidence, not only that nearly all 

 parts of the globe experienced extensive vertical movements dur- 

 ing the Tertiary period, but also that the grand continental oscil- 

 lations do not belong wholly to the past ; for great changes in the 

 elevation of the continental masses, and in the relative distribu- 

 tion of land and sea, are now in progress. Geologists are famil- 

 iar with this evidence, and I will notice only a single example. 

 Perhaps the most striking instance of change of* continental level 

 now in progress is that afforded by the western half of South 

 America. It was long ago demonstrated, by the observations of 

 Darwin and others, that this part of the earth's crust is now ris- 

 ing slowly, yet at a rate which is measurable from century to 



PROCEEDINGS B. S. N. H. VOL. XXII. 29 JULY. 1884. 



