GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 33 



in Precambric time. Because of their early crystalline condition, 

 these gneisses would have undergone fewer changes and a relatively 

 lesser degree of metamorphism than the sediments which overlay 

 them, during the mountain building process of Ordovicic time. Such 

 changes as they underwent from this cause should, however, show 

 some correspondence with those in the younger rocks, as is perhaps 

 afforded in the apparent lesser degree of metamorphism at the west. 

 This difference is not, however, noticeable in the field unless the more 

 clearly '' bedded " strata in the bed of Mount Beacon brook and the 

 more clearly definable nature of the altered Precambric gneisses of 

 the Glenham belt are indications of it. 



An examination of the thin sections of the gneissoid types from 

 the Glenham belt entirely supports the assertion that these rocks are 

 members of the Highlands gneiss series. In mineralogy, texture and 

 metamorphic characters they are entirely similar. The thin sections 

 of the more characteristic types of this belt afford the clue to their 

 interpretation and seem to show their original nature. They also 

 carry characteristic strain effects. 



FAULTS IN THE GNEISSES 



During the Green mountain uplift the Precambric gneisses appar- 

 ently buckled somewhat, but seem to have yielded chiefly by break- 

 ing. These faults greatly complicate the problem of the configura- 

 tion of the Precambric land mass while the quartzite was being laid 

 down. 



Beginning at the west, the first fault is that shown by the Glenham 

 belt. A reversed or thrust fault has thrown the gneisses against the 

 slates on the west and south. Evidently the slates were folded and 

 overturned and then overridden by the older rocks. The strati- 

 graphic displacement necessary to elevate the Precambric into con- 

 tact with the slates must have been an extensive one. Apparently 

 at Vly mountain the upthrust was greater, resulting in the elevation 

 of the mountain mass above the main portion of the belt and caus- 

 ing the transverse break between the two. That Vly mountain 

 is not mainly an erosional feature is indicated by its relationships. 

 It forms an isolated block which is faulted against the slates on the 

 west, north and east. The transverse fault on the south involved 

 the limestones which were bought against the slates on the east 

 of them. The gneiss and limestone form the upthrow as a result 

 of reversed faulting, both resting against the slate. The gneiss 

 apparently also moved with reference to the limestone. Pro- 



