24 NEW YORK STATE MUSEUM 



other two. The purpHsh color of the darker rock is plainly due to 

 the abundant magnetite. 



Though apparently greatly decomposed, these rocks are tirm 

 and compact in the hand specimen. The magnetite is not altered 

 into hematite or limonite. The conditions suggest that the altera- 

 tion of these rocks dates back to an epoch preceding the deposition 

 of the basal quartzite, which, as the proximity of this formation 

 shows, formerly covered the gneisses, probably until glacial time. 



East of the east road into the mountains the quartzite has been 

 dropped by a fault. It extends farther to the south than on the 

 west of the road, partly on this account and partly because of a 

 syncline at this point. No peculiar variations were noted in cross- 

 ing the Hook spur to East Fishkill Hook. The southward exten- 

 sion of the quartzite leaves comparatively few outcrops outside the 

 thickly-wooded area of the spur. 



Shenandoah mountain.^ Above the drift-covered slope of the 

 quartzite, along the northwestern slope of the mountain, dark, 

 micaceous gneisses were noted in conspicuous ledges. Along the 

 road from the East Hook to Hortontown, these were succeeded near 

 the summit of the mountain by a light granite interbedded with the 

 gneisses and estimated to be from forty to sixty feet thick. I have 

 called this the Shenandoah mountain granite. With the exception 

 of one or two quartzitic members, the usual succession of the 

 gneisses is crossed in going from the granite " stratum " across the 

 mountain to Hortontown. On the whole, the micaceous types 

 seemed more abundant. Outcrops are numerous along the road and 

 in the fields on each side. 



The age of the granite can not be affirmed. It appears to have 

 the strike of tlie adjacent gneisses ; but it did not prove possible to 

 trace it more than a few hundred feet. The quartzite formation, or 

 its possible equivalent, was not found resting on the granite, so that 

 its age could not be definitely assigned by showing an unconformity. 

 If thrust up into the gneisses at the time of their folding, it has 

 escaped foliation. It probably belongs to the Precambric series. If 

 so, the absence of foliation indicates that Postcambric movements 

 did not contribute to the characteristic foliation of the gneisses. 



The eastern gneiss mass. The rocks along the northwestern 

 base of the eastern gneiss mass in some cases suggest a continuation 

 of those of Shenandoah mountain. 



1 The spur next east is locally known as Shenandoah mountain, from 

 the hamlet of that name at its northern termination. The Shenandoah 

 of the map is East Fishkill Hook. 



