Crosby.] 448 [November 7, 



that areas where thick deposits of sediments are formed usually 

 are, and, as a rule, must be, areas of subsidence. 



Herschel started with this datum of the science, and added the 

 following : the rising of the isogeotherms softens and weakens 

 the hydrated crust ; and the weight of increasing sediments bears 

 it down. But it is now generally conceded by geologists that 

 subsiding sea-floors are the cause and not the consequence of 

 thick deposits of sediments. 



Although the true theory of elevation and subsidence must be 

 one recognizing a variety of causes ; yet the mere thickening and 

 thinning of the crust by expansion and contraction, and the soft- 

 ening of its lower portions, do not explain the grand facts of the 

 rising and falling of continents and ocean-floors. In short, the 

 magnitude of the phenomena compel us to suppose that, in many 

 cases, elevation and subsidence are due to the bending of the 

 crust through its entire thickness. But this supposition involves 

 the farther supposition of a plastic zone beneath the crust. And, 

 as a matter of fact, the great majority of geologists, in spite of 

 the reaction from the hypothesis of a liquid globe, do admit the 

 existence of a relatively soft and plastic layer as the only satis- 

 factory explanation of the extensive vertical movements of the 

 earth's surface. This point is even conceded by those who, like 

 Professor Dana, stand most strongly, in their theories of conti- 

 nents, for a globe continuously solid from centre to circumference. 



In the American Journal of Science (3), vol. 6, p. 7, Professor 

 Dana says : " The Appalachian subsidence in the Alleghany re- 

 gion 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 contrac- 

 tion in the strata themselves or the terranes underneath. But 

 such a subsidence is not possible, unless seven miles — that is, 

 seven miles in maximum depth and over a hundred in total 

 breadth — of something were removed, in its progress, from 

 the region beneath." In other words, " there existed, un- 

 derneath a crust of unascertained thickness, a sea or lake of 

 mobile (viscous or plastic) rock, as large as the sinking region." 

 And it is also stated that " this under- Appalachian fire-sea proba- 

 bly dated back to the era of the general fluidity of the earth." 



