S2 NEW YORK STATE MUSEUM 



features are plainly far from decisive. On the whole, the sections 

 are less satisfactory than the field outcrops; but so far as they 

 go they sustain the uncertainty of. the field examination. 



If these gneisses are mainly altered sediments they have been so 

 thoroughly crystallized that they now often closely resemble igneous 

 types. The hornblendes in their relation to the feldspars sometimes 

 indicate a formation in the usual order of crystallization from a 

 magma. If mainly of igneous origin, these gneisses were greatly 

 squeezed in their formation and would now be more properly desig- 

 nated gneissoid eruptives than eruptive gneisses. In either case the 

 primary minerals (that is, those plainly belonging to the last change 

 that affected the whole rock) and their essential arrangement are of 

 contemporaneous origin. 



So far as examined, the sections are entirely free of the minerals 

 usually found in areas of profound dynamic metamorphism. It is, 

 of course, impossible to tell how many complete metasomatic or other 

 changes these rocks may have undergone, but there appear to be no 

 traces of any antecedent generations of minerals. 



The sections sustain the belief that the primary features of the 

 gneisses, as a whole, are of very ancient character and of Precam- 

 bric age. They show, on the other hand, many evidences of subse- 

 quent metamorphism. 



This later metamorphism is shown in the sections in several ways, 

 but chiefly as pressure effects. In almost all cases the quartz crystals 

 show pronounced strain phenomena, such as strain shadows and 

 wavy extinction, and are often cracked. The plagioclases almost 

 always show pinched-out, bent or broken lamellae. Fractures and 

 long cracks are common. In places where the gneiss evidently had 

 undergone an early alteration, the rock was indurated and occasion- 

 ally new minerals formed. Some molecular movement is indicated 

 by chloritic fillings, disseminated magnetite and secondary quartz 

 injected into the feldspars. Some biotite very clearly belongs to this 

 later metamorphism. 



Some of the sections from the Bald hill gneiss and those in the 

 bed of Mount Beacon brook show fewer apparent strain effects 

 than those from the spurs farther east, which may be interpreted 

 as the expression within these rocks of a somewhat lesser degree 

 of metamorphism at the west. The conclusion that the primary 

 gneissic characters were changed very little in Postcambric time 

 seems inevitable. As the field relations show the gneisses had reached 

 practically their present crystalline condition and gneissic structure 



