EUREKA QUARTZITE. 55 



McCoy's Ri(l<»-e it has heiMi ([uan-icd tor flnxini;- [iiirposcs at the siiicltini;- 

 t'uruaces, the rock yiel(liu«>' nearly two dollars in <>-ol(l ])er ton, which paid 

 for hauliiij^'. Whether the ofold is of priiuary origin in the (piartzite or 

 whether it was derived from some vent carrying mineral matter in solution 

 has never been determined. The locality Avhei-e the rock was (juarried is 

 situated near the Hoosac fault, and in close ])roximity to ore bodies. 



The ridge extending southwest from Castle Mountain shows a line 

 body of the Eureka (jnartzite, the southern escarpment of which exjjoses a 

 section 300 feet in thickness. Numerous specimens collected at intervals 

 across the quartzite were subjected to microscopic examination. All the 

 u])per portion of the rock proved to be an exceptionally pure and fine 

 (piartz, the grains averaging between 0'02 and 0-03 millimeters in size, 

 with a granitoid structure ; that is, the grains did not show rounded 

 outlines, but instead presented irregular shapes that titted into each other 

 and tirmly crystallized together without tine groundmass between them. 



The quartzite is free from impurities but full of fluid inclusions with 

 moving bubbles, some of them evidently liqui(l carbonic acid. The minute 

 fluid cavities appear white in incident light. An examination of the quartzite 

 indicated that the entire rockmass had undergone a recrystallization of 

 the material and was not by any means a sinq)le solidification and packing- 

 together of quartz grains. In other words, it is a true quartzite and not a 

 compact sandstone, hardened by superincumbent rock. Even under the 

 microscope the rock appears to cany but little oxide of iron. Toward 

 the upper part of the formation the microscope detects increasing numbers 

 of needles and grains of iron o.xide, accounting for the change of color 

 both in the unaltered rock and on the weathered surfai'es of the larger 

 detached blocks. Particles of calcite also begin to appear some distance 

 beneath the Lone Mountain limestone, associated with the quartz gi'ains, 

 while at the base of the quartzite there is a very decided increase in the 

 amount of lime present. 



Although not differing materially from those observed elsewhere, the 

 most satisfactory section across the quartzite was made just west of Castle 

 Mountain. Here the quartzite presents a perpendicular cliff, 300 feet in 

 thickness, resting horizontally on the I'ogoni]) liuiestone. The subjoined 



