6o NEW YORK STATE MUSEUM 



sources. In this case the original is judged to be a representative of 

 the Grenville series and the invading material is judged to be the 

 Canada Hill granite. (Photomicrograph of no. 395 shows a typical 

 field.) The invading magma is represented chiefly by microcline and 

 the original rock material by the other more irregularly disturbed 

 and confused looking grains. The older minerals have multitudes of 

 inclusions and modification products whereas the invading minerals 

 are perfectly fresh (plate 33). 



Metamorphics of doubtful age. Three formations are classitied 

 under this head in preference to connecting them more definitely in 

 the historical sequence. They form a series including: (a) the 

 Manhattan schist, (b) Inwood limestone, (c) Lowerre quartzite. 



The first two are largely developed in this quadrangle and the 

 last one very erratically or not at all. These formations can be 

 traced from New York City, which is the type locality, northward to 

 the Highlands into the southeast quarter of this quadrangle. Here 

 they have a typical petrographic development, but their prominence in 

 the field is much obscured by the heavy drift cover. The Cortlandt 

 series also cuts into the area which would otherwise be occupied by 

 these formations and eliminates about 20 square miles. 



,No rocks of this type are found west of or northwest of Peek- 

 skill and the abrupt termination of so great a series with no rem- 

 nants of similar habit beyond this border is one of the striking 

 facts, and introduces one of the largest unsolved structural prob- 

 lems of the region. It has been suggested that this series is the 

 equivalent of the Hudson River-Wappinger-Poughquag series, but 

 the authors feel that this has not been proved. In the lack of a 

 better classification of age relations and, because particularly of the 

 very strikingly different petrographic habit of these formations as 

 compared with the other series they prefer to treat them in a separate 

 class and are willing to consider their age doubtful. 



a Manhattan schist. The Manhattan formation is everywhere a 

 schist of very complete recrystallization. In structural make-up it is 

 strongly foliated and of dominantly micaceous composition. In some 

 cases it is almost wholly mica with a light pearly mica predominating, 

 but it varies from this to a strongly quartzose rock, essentially a 

 quartz-mica-schist and in some of these cases black mica is fairly 

 prominent. Pearly mica, however, is the most characteristic single 

 mineral. 



At occasional points strongly hornblendic schists are developed 

 from former igneous intrusions of essentially diabasic composition, 

 but these are not largely developed in exposures of this quadrangle. 



