Barrel! — Growth of Knowledge of Earth Structure. 157 



nature of the folds as already made known by the Rogers. 

 Dana pointed out in reply to Hall that the folding did not 

 correspond to the requirements of Hall's hypothesis, 

 especially as the folding took place not during, but after 

 the close of the vast Paleozoic deposition. Dana states 

 in conclusion on Hall's hypothesis (42, 209, 1866) that 

 "It is a theory of the origin of mountains with the origin 

 of mountains left out." 



The Theory of Geosynclines and Geanticlines. 



The fact that systems of folded strata lie along axes of 

 especially thick sediments and that this implied subsi- 

 dence during deposition was Hall's contribution to geo- 

 logic theory, but curiously enough he failed, as shown, to 

 connect it with the subsequent nature of mountain fold- 

 ing. He did not see why such troughs should be weak to 

 resist horizontal compression. The clear recognition of 

 this relationship was the contribution of Le Conte, who 

 in a paper on "A theory of the formation of the great 

 features of the earth's surface" (4, 345, 460, 1872), 

 reached .the conclusion that "mountain chains are 

 formed by the mashing together and the up- swelling of 

 sea bottoms where immense thicknesses of sediment have 

 accumulated. ' ' 



As to the cause why mashing should take place along 

 troughs of thick sediments Le Conte adopts the hypothe- 

 sis of aqueo-igneous fusion proposed independently long- 

 before by Babbage and Herschel and elaborated into a 

 theory of igneous rocks by Hunt. Under this view, as the 

 older sediments became deeply buried, the heat of the 

 earth's interior ascended into them, and since they 

 included the water of sedimentation a softening and met- 

 amorphism resulted. Dana had shown, however, six 

 years previously (42, 252, 1866), as the following quota- 

 tion will indicate, that metamorphism of sediments 

 required more than deep burial and that no such weaken- 

 ing as was postulated by Herschel had occurred : 



"The correctness of Herscliel's principle cannot be doubted. 

 But the question of its actual agency in ordinary metamorphism 

 must be decided by an appeal to facts ; and on this point I would 

 here present a few facts for consideration. 



The numbers and boldness of the flexures in the rocks of most 

 met amorphic regions have always seemed to me to bear against 



