HIGH-GRADE SILICA MATERIALS 



Part i 



THE POUGHQUAG QUARTZITE (LOWER CAMBRIAN) IN 

 WESTCHESTER COUNTY 



General description. The quartzite in this area lies about three- 

 fourths of a mile northwest of the center of the city of Peekskill, 

 along the southwest margin of Peekskill creek; it extends from a 

 point near the tracks of the New York Central Railroad at the mouth 

 of the creek, in a northeasterly direction to within approximately 

 500 feet of the state road running north from Peekskill and cross- 

 ing Peekskill creek. A flat-topped ledge with almost vertical walls 

 10 to 12 feet in height in places, may be traced for 1200 or 1500 

 feet. The beds appear to be slightly overturned to the northwest ; 

 the overlying Wappinger limestone has been partially stripped by 

 erosion, so that the ground falls away in a steep descent to the creek. 

 On the other hand, a more or less fractured and crushed zone occurs 

 between the quartzite and the gneiss, and weathering has operated 

 along this line of weakness as well, thus imposing upon the much 

 more resistant and steeply dipping quartzite the dikelike form 

 mentioned. , 



The best rock is contained in this ledge; in other places in the 

 immediate vicinity the rock is much less pure, and occasionally so 

 strongly schistose as to be essentially a quartz schist (figure 4). 



The rock in the dikelike ledge is a massive, strongly jointed, fine 

 textured, highly indurated, grayish white to grayish pink, hard, 

 tough quartzite, inclined to be somewhat streaked, and weathering 

 on exposed faces and along joints to splotchy, yellowish brown tones 

 very erratically distributed. 



Lithologic character. Thin sections of various samples of the 

 rock taken at intervals along the strike, exhibit the following 

 features : 



Angular, jagged and elongate grains, ranging from o.io mm to 

 2 mm in diameter, and averaging 0.30-0.40 mm ; well, and in some 

 cases even intricately interlocked, with no traces left of original 

 grain outlines. The dominant component is, of course, quartz; 

 according to the chemical analysis (table i), the percentage of silica 

 in mixed samples of this rock is 95.51 per cent. Not all the silica 

 is present as quartz, however, as numerous feldspar grains form 

 the next most abundant mineral present. 



