KNOWLEDGE OF EAETH STEUCTURE 177 



evidences that the successive formations were deposited 

 in shallow water. It suggested to them that the weight of 

 the accumulating sediments was the cause of subsidence, 

 each foot of sediment causing a foot of down sinking. 

 This idea has continued to run through various text 

 books in geology for half a century, yet Dana early 

 saw the fallacy and in 1863 in the first edition of 

 his Manual of Geology (p. 717) states " whether this 

 is an actual cause or not in geological dynamics is 

 questionable." In 1866 in an important article on 

 "Observations on the origins of some of the earth's 

 features," Dana deals more fully and finally with 

 this subject (42, 205, 252, 1866). He shows that such an 

 effect of accumulating sediment postulates a delicate 

 balance, a very thin crust and no resistance below. If 

 such a weakness were granted it would be impossible for 

 the earth to hold up mountains. Furthermore such sub- 

 sidence was not regular during its progress and finally 

 in the long course of geologic time gave place to a reverse 

 movement of elevation. 



Hall had pointed out the fact that the sediments were 

 thickest on the east in the region of mountain folding and 

 thinned out to a fraction of this thickness in the broad 

 Mississippi basin. Hall argued that the mere subsidence 

 of the trough would produce the observed folding and 

 that the folding was unrelated to mountain making or 

 crustal shortening. In supposed proof he cited the fact 

 that the Catskills consist of unfolded rock, are higher 

 than the folded region to the south, and nearly as high as 

 the highest metamorphic mountains to the east. 10 Hall 

 and all his contemporaries were handicapped in their 

 geological theories by a complete inappreciation of the 

 importance of subaerial denudation. For subscribing to 

 these errors of their time even the ablest men should not 

 be held responsible. Hall was the most forcible person- 

 ality in geology in his generation. His contributions to 

 paleontology were superb. His perception of the rela- 

 tion existing between troughs of thick sediments and 

 folded structures was a contribution of the first import- 

 ance; yet in the structural field his argument as to the 

 production of the Appalachian folds by mere subsidence 

 during deposition indicates a remarkable inability to 



