438 Ilohbs — Geological Structure of the 



deeply involved in the controversy to settle them. Walcott, 

 by his discovery of fossils in beds hitherto supposed to be bar- 

 ren of them, was able to review the early questions of strati- 

 graphy and to end forever the Taconic Controversy. 



The problem of the areal delimitation of horizons within 

 the New England area and the determination of the geological 

 structure of the region remained, and the early dissensions had 

 at least shown the necessity for a much more thorough survey 

 of the region than any which had been attempted. The work 

 of the U. S. Geological Survey within the region, begun before 

 that of Dana had been completed, was therefore taken up with 

 a plan more comprehensive than any other and on a scale more 

 nearly what was demanded by the conditions of the problem. 



In view of the difficulties which have been encountered in 

 the progress of this work and the methods of investigation 

 which experience has shown to be of value, it is well to con- 

 sider for a moment the fundamental assumption of Dana in 

 his study, and also the dominant idea maintained throughout 

 all the earlier work of the U. S. Geological Survey. Dana in 

 his later papers summarizes the general geological character- 

 istics of the Taconic region and says of the structure : 



"The rocks owe their positions to a system of flexures, and the 

 folds are to a large extent overthrust folds."* 



It is evident from his sections that it was not Dana's intention 

 to express by the term "overthrust folds" ruptured folds, but 

 folds with inclined axial planes. In his summary he makes no 

 mention of faults, and though a fault is indicated in one or two 

 of the sections, it is evident that faulting hardly figured in his 

 conception of the structure of the region. 



Essentially the same opinion was held by Pumpelly, who 

 was in charge of the work of the U. S. Geological Survey dur- 

 ing the earlier years, when the Hoosac-Greylock province in 

 northwestern Massachusetts and southern Vermont was sur- 

 veyed. With him, however, the supposed absence of faulting 

 within the area was clearly expressed as the reason for assum- 

 ing a deformation wholly by folding. In his report he says : 



" It was evident that our first and hardest work would be to 

 find the key to the structure of the region. For this purpose I 

 sought a region where the western edge should present, instead 

 of a straight line, as many bay-like curves as possible, and where 

 the structure of the ridge itself should show folds with pitching 

 axes. I hoped in such a region to eliminate the difficulties intro- 

 duced by possible faults, as well as the temptation to infer their 

 existence; and also, in the case of pitching folds, to get through 



* James D. Dana ; On Taconic rocks and stratigraphy, with a geological 

 map of the Taconic region. This Journal (3), xxix, 209, 1885. 



