58 NEW YORK STATE MUSEUM 



material that they can not well be treated as a separate member. The 

 most pronounced occurrence of this sort is in a belt beginning at 

 Peekskill and extending in a somewhat broken way toward the 

 northeast and again in the vicinity of Oscawana extending also 

 somewhat brokenly toward the northeast. The most distinctly indi- 

 vidual occurrence is at Peekskill, where a belt about 1000 feet in 

 width lies between simple Grenville on the one side and a granite on 

 the other. 



Undoubtedly these hornblendic plagioclase rocks are related to 

 the Pochuck diorite type in origin and, wherever they can be sep- 

 arately mapped, they should be indicated as Pochuck gneiss. This 

 can be done at a few places such as the belt at Peekskill extending 

 to the northeast, but a much larger number of occurrences are on 

 such a small scale and so intermixed with material judged to be of 

 other sources and relations that they have not been separately 

 mapped. This is particularly true of considerable areas toward the 

 east side of the quadrangle where the hornlilendic varieties of 

 rock are abundantly intermixed with remnants of the Grenville. 

 We have indicated a large area as Grenville mixed with granite 

 but have not found a way of indicating its Pochuck intermixture 

 in all cases. 



This rock carries a large proportion of either hornblende or 

 pyroxene or both, usually also biotite, an abundance of plagioclase 

 and varying amounts of quartz. The accompanying photomicrographs 

 illustrate the normal appearance. (Plates 22, 23 and 24.) 



b Magnetite schist. Some of ihe finer grained rocks which carry 

 magnetite in considerable portion have the general appearance of 

 schists, but microscopically they show little foliation habit. The con- 

 stituents are generally pyroxene, sometimes hornblende, abundant 

 quartz and feldspar and magnetite. The proportions vary greatly 

 and accessories are sometimes prominent, especially apatite. They 

 are usually impregnation and replacement products representing a 

 phase of igneous invasion. The general structural habit and mineral 

 relation is shown in the accompanying photomicrographs. (Plates 

 25 and 26.) 



c Quartzitic gneiss. Occasional occurrences of very limited extent 

 have the hand specimen appearance of recrystallized quartzites and 

 were at first marked so in the field. Microscopic examination shows 

 that they are not by any means so simple and are evidently not 

 quartzites at all. Instead they are judged to be extreme dififerentia- 

 cion products of some injection unit high in qiuartz, (Plate 27.) 



