EUREKA QUARTZITE. 55 



McCoy's Ridge it has been quarried for fluxing purposes at the smelting 

 furnaces, the rock yielding nearly two dollars in gold per ton, which paid 

 for hauling. Whether the gold is of primary origin in the quartzite or 

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

 has never been determined. The locality where the rock was quarried is 

 situated near the Hoosac fault, and in close proximity to ore bodies. 



The i-idge extending southwest from Castle Mountain shows a fine 

 body of the Eureka quartzite, the southern escarpment of which exposes a 

 section 300 feet in thickness. Numerous specimens collected at intervals 

 across the quartzite were subjected to microscopic examination. All the 

 upper portion of the rock proved to be an exceptionally pure and fine 

 quartz, the grains averaging between - 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 fitted into each other 

 and firmly crystallized together without fine groundmass between them. 



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

 moving bubbles, some of them evidently liquid 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 simple 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 oxide, accounting for the change of color 

 both in the unaltered rock and on the weathered surfaces of the larger 

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

 beneath the Lone Mountain limestone, associated with the quartz grains, 

 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 Pogonip Ihnestone. The subjoined 



