426 IHE JOURNAL OF GEOLOGY. 



cock's Bear- Mountain locality, are three of the most instructive 

 sections, where. the contact relations of the two series are shown. 

 All these sections show the relations of the two series in apparent 

 structural conformity brought about by dynamic movements 

 exercised throughout the rocks as a whole, but having a maxi- 

 mum obliterative effect immediately at the base of the conglom- 

 erate, since at this point the underlying rocks were best condi- 

 tioned to record such action. Speaking of the transitional beds 

 on Hoosac Mountain, between the Lower Cambrian quartzite- 

 conglomerate horizon and the granitoid gneiss, Mr. Pumpelly 

 writes as follows: ^ "This unabraded zone of crystallme rock," 

 (reference is made here to the zone of semi-disintegrated rock on 

 which the conglomerate was deposited unconformably) " which 

 had its rigidity weakened by beginning disintegration, would, 

 under folding, pressure, and metamorphism, show on the one 

 hand a perfect and true transition into the parent crystalline rock, 

 and on the other hand pass into the much younger beds through 

 the similarity of the constituents derived from it ; and an appar- 

 ent conformity would be forced upon the whole series, and the 

 time break would be masked by the foliation induced by the 

 shearing action due to a slipping movement." An interpretation 

 which so satisfactorily accounted for the transition obtaining on 

 Hoosac Mountain can be as well applied to the transitions in 

 Vermont at the base of the conglomerate, only here the terranes 

 below are of a very variable character, and in a great part were al- 

 ready possessed of a gneissic habit which by-rearrangement would 

 even more readily take on the lamination of the rocks above. 

 Wherever the conglomerate gneiss is found on the west side of 

 the range a perfect transition to the lower rocks always exists, 

 and all evidence of a discordance, such as obtains in more 

 modern rocks of necessity must have been obliterated. It is 

 thus seen that criteria applicable for the detection of more recent 

 time-breaks have but little value where the rocks have been 

 subjected to such powerful and repeated orographic disturbances, 



' The Relation of Secular Rock-Disintegration to Certain Transitional Crystalline 

 Schists, R. Pumpell}', Bull. Geol. Soc. of America, Vol. II., p. 215. 



