GENERAL STRUCTURE AND CORRELATION. 1 



AGE AND STRUCTURE. 



In beginning work on the geology of New England, two facts were 

 apparent — that from the Green mountains eastward the rocks were all highly 

 metamorphosed and crystalline; and that only in two or three localities had 

 fossils been found, and in these places the rocks were so much disturbed 

 that it seemed hopeless to use them ;is starting points for the general work. 

 I became convinced that our hopes of determining the age of the New 

 England rocks lay in using the Green mountains as a bridge. In following 

 this plan we were immediately met by the fact that on the main ridge — our 

 proposed bridge — the rocks are not only highly metamorphosed and their 

 structure the reverse of simple, but that the western edge of the ridge marks 

 an abrupt lithologic change between the character of the rocks of the 

 mountain and those of the valley, with the exception of the younger schists, 

 which in places cap both the axial range and the valley hills. On the west 

 the great limestone and an underlying great quartzite come eastward to the 

 base of the mountain, while a careful reconnaissance showed no trace of these 

 rocks as such upon the mountain, nor of such a combination on the eastern 

 side. 



This difficulty, which met the earlier surveys, had led to various 

 hypotheses in which faults and overturns played an important part. And 

 while the rocks of this main ridge were assigned by different eminent 

 geologists to ages ranging from the Sillery 1 to Huronian and Laurentian, 2 

 the residuum of opinion has been of late in favor of Archean or at least 

 pre-Cambrian age. The problem was undoubtedly to., difficult to be 

 solved without more ample means than were at the disposal of our pre- 

 decessors. 



It was evident that our first and hardest work would be to find the key 

 to the structure of the range. 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. 1 hoped in such a region to eliminate the difficul- 



1 Logan colors them as Sillery on the Geological map of Canada. 18(>6. 



*C. 11. Hitchcock: geological sections across New Hampshire and Vermont. Hull. Am. Jlus 

 Nat. Hist., vol. l, New York, 1884 



