REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1900 rl55 



in face of competition of the Yonkers gneiss quarries only be- 

 cause of shorter haul and the fact that removing the stone was 

 raising the value of the lots. 



Fordham, Westchester co. Fordham gneiss was formerly quar- 

 ried (8) west of the Harlem railroad, south of Fordham, in the 

 uppor beds, much of the product being used for railroad ballast. 

 It has also been quarried for road metal, for which purpose it 

 ^'9 not well suited, at various points in Westchester county, but 

 none of these small quarries are now working and none were 

 ever of much importance. 



Yonkers gneiss 



The Yonkers gneiss is a well foliated gneissoid granite, very 

 variable in color, grain, and economic value, but quite uniform 

 petrographically. It is composed essentially of quartz, biotite, 

 microcline, and sheared orthoclase. Normal orthoclase is not 

 abundant, while plagioclase is very rare. Hornblende is the 

 most common accessory, being very abundant in certain seams, 

 and rarely entirely absent from a slide. Titanite is very fre- 

 quent. A thin section is shown on pi. 48. 



The color of the Yonkers varies from a li^ht 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 them- 

 selves 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. 



The difference in color is of importance economically. The 

 red forms decay readily while the blue, though often becoming 

 stained yellow by iron, do not appear to disintegrate. The 

 writer has not been able to follow up this investigation as far 

 as he could have wished, and the discussion in this paper should 

 be regarded as merely preliminary to a more detailed presenta- 

 tion of the subject. 



The principal area of Yonkers gneiss is located in the southern 

 part of Westchester county. It extends from near Van Cort- 



