GEOLOGY OF THE HAYSTACK STOCK 203 



the Cambrian quartzite and the limestone. The sills in the Cambrian 

 are characteristic of this formation over wide areas and are not to be 

 regarded as apophyses of the Haystack stock; further, they are more 

 siliceous in composition than the average of the stock, and they are 

 thicker away from the stock than near it. 



There is no conclusive evidence that the Haystack stock ever 

 reached the surface. It is of later age than the acid breccia, and 

 appears to have extended to near the top of this formation, but it 

 may have been deeply buried under the basic breccia, which has a 

 thickness of 2,000 feet or more in Hell Roaring Creek near by. Since, 

 however, its composition is near that of the basic breccia, it is possible 

 that it formed a channel through which a portion of this breccia 

 reached the surface. 



Appearance. — The peripheral facies of the Haystack stock is dark, 

 fine-grained, and contains a few inconspicuous anhedra of feldspar 

 and biotite. A short distance toward the center from the periphery, 

 it becomes of lighter color and coarse grain, and is composed of 

 feldspar, quartz, hornblende, and mica. Still farther from the border, 

 it becomes much more coarsely crystalline, of darker color, and is 

 composed of biotite, pyroxene, and magnetite. In the more basic 

 facies of the central portion of the stock the ferro-magnesian minerals 

 approximately equal the light-colored constituents. The various 

 facies grade one into the other except in rare cases where locally there 

 is a rather sharp contact between them. A notable example occurs 

 near the wagon road a few rods south of the divide between West 

 Basin Creek and East Basin Creek, where dark gabbro of medium 

 grain appears to cut the coarser, lighter gabbro, but a few feet away 

 from this contact these two rocks, traced continuously between, 

 grade into each other as elsewhere. The various rocks of the stock 

 are always massive, and do not show evidence of mashing. Near 

 the border a few fragments of surrounding rocks are present, but 

 these are rare or wanting in the interior of the stock. There has been 

 fracturing since the solidification of the stock and it is traversed by 

 joints in several directions. Along some of these fractures the sur- 

 rounding rock is extensively altered, but this alteration is local and 

 the rocks of the stock are for the most part fresh. 



At a number of localities veins of coarse-grained granite cut the 



