122 NEW YORK STATE MUSEUM 



seen in acid gneisses. Examples of what appears to be crushed and 

 sheared gneiss are frequently observable in the field but they are 

 probably the result of viscous flowage of the magma. 



The granite outcrops in several areas. The principal belt within 

 which most of the quarries are situated parallels the Bronx river 

 and Harlem Railroad from a point a little south of Mount Vernon 

 to Hartsdale, near White Plains. The outcrop lies along a series 

 of hills and ridges between the Bronx and the parallel valleys of 

 Tibbitt and Troublesome brooks. Its surface shows only moderate 

 relief, the highest elevations slightly exceeding 300 feet, with in- 

 tersecting notches and cross-valleys whose bottoms mostly are be- 

 tween 100 and 200 feet. The main intrusion is nearly 10 miles 

 long, but not much over one-half of a mile wide. This form doubt- 

 less results from a sill or sheetlike intrusion of the original granite 

 which penetrated the sedimentary formations of the Fordham along 

 the bedding planes and has since been upturned so as to afford a 

 longitudinal section. 



A second area of the Yonkers occurs along the axis of the main 

 belt farther north, near Valhalla and the Kensico reservoir. This 

 has not been so actively worked as a source of building stone. 

 There are a few quarries, however, that have been operated at 

 different times, mainly to supply foundation material, including that 

 used in the Kensico dam. 



General characters. The Yonkers granite varies more or less in 

 physical structure and appearance. This observation applies even 

 to the limited area of a single exposure, where occasionally the char- 

 acteristic thinly foliate rock may be seen grading over into a quite 

 massive one. There is little variation, however, in respect to the 

 mineral composition, and the whole rock mass is c[uite free from 

 segregations and inclusions. The quarry sites in most instances 

 have been selected with a view to uniformity of the material 

 which is obtainable to a fair degree. Eckel ^ describes the general 

 features of the Yonkers as follows : 



The color of the Yonkers gneiss varies from a light blue to a 

 rather deep red. This variation is partly due to the fact that the 

 blue grades in most cases contain more quartz and less feldspar. 

 A much more potent cause, however, is that the feldspars themselves 

 are either red or bluish. This difference in color is not due to a 

 difference in the feldspar species, as the microcline and sheared 

 orthoclase appear in both the red and blue Yonkers, and in about 

 the same relative proportions. 



1 The Quarry Industry of Southeastern New York. N. Y. State Mus. 

 Rep't 54, 1902, p. 155- 



