AMKKKAN GEOLOGY — -DECADE OF 1850-185.9. 501 



numerous .smaller synclinal and anticlinal axes. The greater amount 

 of compression above or stretching below along the line of maximum 

 thickness of the sediments would account for the gradual decline 

 toward the margin of the major syncline or the evidences of fracture 

 and distortion. This, he thought, afforded a partial explanation of 

 the fact that mountain elevations in disturbed regions bear, in their 

 altitude, a much smaller proportion to the actual thickness of the for- 

 mation than do the hills in undisturbed regions; and, further, that since 

 in the formation of an anticline the beds are weakest at the ridge and 

 become more liable to denudation, such arc often worn down to form 

 low ground or even deep valleys, while the synclinal arches, being 

 protected in the downward curvings of the beds, may remain to form 

 the prominent mountain crests, as is observable in the southern Appa- 

 lachians. It now here seemed to him that folding or plication had con- 

 tributed to the altitude of the mountains, but rather that the more 

 extreme the plication the greater had been the general degradation of 

 the mass wherever subjected to denuding agencies. 



The chief elevation of the Appalachian chain, he argued, was conti- 

 nental and not of local origin, and the present mountain barriers to 

 him were but the visible evidences of the deposits upon an ancient 

 ocean bed, while the determining cause of their elevation existed long- 

 anterior to the production of the mountains themselves. 



At no point nor along any line between the Appalachian and Rocky Mountains 

 could the same forces have produced a mountain chain, because the materials of 

 accumulation were insufficient, and though we may trace what appears to be the 

 gradual subsiding influences of these forces, it is simply in these instances due to the 

 paucity of the material upon which to exhibit its effects. 



Referring to the amount of metamorphism which these rocks had 

 undergone, Hall thought that we must look to some other agency than 

 heat for the production of the phenomena, and that the "prime cause 

 must have existed within the material itself; that the entire change 

 was due to motion or fermentation and pressure aided by a moderate 

 increase of temperature, producing chemical change." Just what is 

 meant by this it is difficult to say, but, inasmuch as Hall seems to have 

 been in consultation with Sterry Hunt, it is safe to assume that it was 

 intended to include all possible causes which future investigation 

 might show to have been operative. 



These views of Hall seem not to have been favorably received at 

 first by the American geologists, and were facetiously referred to 

 by Dana as proposing a system of mountain making with mountains 

 left out. To this Hall very justly replied that he had not intended 

 to offer any new theory of elevation, nor to propound any principle 

 as involved beyond what had been suggested by Babbage and Herschel. 

 What he did intend to imply was that mountain elevation was due 

 to sedimentary accumulation and subsequent continental elevation. 



