120 NEW YORK STATE MUSEUM 



Microscopic character. Feldspar is first in importance as a con- 

 stituent and consists mainly of albite or acid oligoclase with sub- 

 ordinate orthoclase. The individual crystals often show marked 

 zonal structure. Alteration is evidenced by clouding and the de- 

 velopment of muscovite and probably also of kaolin. The quartz 

 is slightly gray or smoky in color. Of the micas, muscovite is 

 equally common with the biotite variety and occurs in original 

 crystals, as well as secondary growths from feldspar. The biotite 

 shows partial change to chlorite. Iron ores are very scarce except 

 for the little limonite that occurs in the exposed part of the granite. 

 The grain may be classed as medium, the coarser particles of feld- 

 spar and quartz attaining a diameter of 10 mm. The interspaces 

 are filled up -with finer interlocking individuals and the texture is 

 very compact. 



Crushing strength. A crushing test performed by Ricketts & 

 Banks on a sample from the quarry, as communicated by J. M. 

 Rudiger, showed an ultimate strength of nearly 21,000 pounds 

 to the square inch. The details are as follows: size of cube, 1.99 

 by 2 by 1.99 inches ; area 3.98 inches ; breaking strain 83,100 pounds ; 

 ultimate strength 20,870 pounds a square inch. The granite is 

 unquestionably strong and durable. 



Roberts quarry 



An exposure of granite occurs in the knob lying just southeast 

 of Jacobs hill and between the Peekskill-Lake Mohegan road and 

 the Catskill Aqueduct. It is more than a mile west of Millstone 

 hill. The knob is of small compass, a few hundred feet in diameter 

 and less than 100 feet high. It has been opened on the southeastern 

 side to supply stone for local construction. The quarry is only 

 about a mile out of Peekskill and appears to be located at the most 

 accessible point of the granite area. 



The quarry cut is about 100 feet long, with two small-sized der- 

 ricks in place. The granite is well jointed along two directions, 

 N. 6o° W., and N. 20 E. but is not sheeted. 



The stone differs considerably in texture and appearance from 

 that exposed in other parts of the area, but the general composition, 

 so far as the nature of the mineral ingredients are concerned, is 

 similar. It has a coarse grain which is made very prominent by the 

 large micaceous aggregates of dark color, whereas the body of 

 feldspar and quartz has the usual light hue. These aggregates 

 formed by intergrowing muscovite and biotite attain a diameter of 

 half an inch ; they are oriented parallel with the rift, and to surfaces 



