Page:The Journal of geology (IA journalofgeology21894univers).pdf/442

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 maximum obliterative effect immediately at the base of the conglomerate, since at this point the underlying rocks were best conditioned 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:^[1] "This unabraded zone of crystalline 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 apparent 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 already 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,