112 NEW YORK STATE MUSEUM 



THE PEEKSKILL OR MOHEGAN GRANITE 



Granite intrusions are found on the borders of the area occupied 

 by the Cortlandt series, which is the name given to an interesting 

 group of basic igneous rocks exposed to the south and east of 

 Peekskill. The Cortlandt series comprises diorites, gabbros, norites, 

 pyroxenites and other types of basic habit, with such relationship 

 as to indicate that they represent the differentiated products of a 

 single deep-seated magma. Their intrusion took place probably as 

 late as Siluric times since the series breaks through and includes 

 portions of the metamorphosed sediments that are classed with the 

 Hudson River series of the Lower Siluric. Their outcrop extends 

 over an area 5 miles in east-west diameter and about 4 miles from 

 north to south, in outline an immense boss. 



The granite exposures are on the north side of the Cortlandt area 

 and immediately adjacent to it. The first outcrop encountered 

 on the west is a mile or so out of Peekskill on the little knob lying 

 between the Lake Mohegan road and the east- west highway, just 

 west of the line of the Catskill Aqueduct. The locality is known 

 as the Roberts quarry. Millstone hill, which lies a mile farther east 

 and south of the east-west highway, is made up in its northern 

 slopes of granite, but is apparently near the contact with the basic 

 rocks of the Cortlandt series which appears on the next prominence 

 to the west. A third place where granite appears in force is across 

 the valley from Millstone hill, on the south and west slopes of a 

 ridge, about a mile south from Lake Mohegan. The Mohegan 

 Granite Company has quarries at this locality. 



In the several exposures which embrace between them an area 

 of 3 or 4 miles, there is naturally some variation in the appearance 

 and composition of the granite, though as a whole the samples from 

 the different quarries exhibit a degree of uniformity which would 

 seem to establish their identity with one and the same intrusive 

 mass. This uniformity is reflected in the predominance of white 

 feldspar, mainly orthoclase, a'lbite and oligoclase, which gives a 

 light tone to the lock wherever exposed, in the presence of both 

 biotite and muscovite, a moderate to small content of transparent 

 quartz, and in the granitic texture which ranges from medium to 

 fine grained. It appears probable that the different quarries are lo- 

 cated on outcrops of a single body which has the Cortland series 

 on the southwest and lies against the metamorphic rocks, including 

 Paleozoic schists, on the remaining border. The exact extent and 

 shape of the mass is somewhat indefinite, as there is a heavy cover- 



