5S2 PROCEEDINGS OF THE WASHINGTON MEETING 



2 of the same plate gives a good general idea of the jointing in the broad portion 

 of a Long granite ridge at Sylvan lake, about 4 miles south of Harney peak. The 

 lake has been formed artificially by throwing a dam across a narrow gorge through 

 which a small stream finds its way. It is evidently not a "rock basin." The 

 granite here contains a large proportion of muscovite, so much in fact that the at- 

 tempt has been made to exploit it commercially. This ridge is comparatively 

 broad and i he forms produced by disintegration and erosion are more rounded 

 than they lire in some other parts of the district. The most striking and interest- 

 ing of the erosion forms are those to be seen in "Cathedral park," a small area 

 about 2 miles southeast of Harney peak, where the narrow ridges of granite have 

 been weathered into a remarkable series of jagged pinnacles, a few views of which 

 are reproduced on plates 54, 55, and 56. The ridges now standing are divided into 

 plates the long diameters of which are parallel with the main system of joints in 

 the region, or about northwest and southeast. The main system of joint planes is 

 crossed at various angles by subordinate planes of fissure. The weathering has 

 been most extensive along the more persistent joints, and the combination has 

 produced the almost endless variety of jagged forms which characterize the Har- 

 ney Peak district. Degradation along strong fracture planes nearly at right angles 

 to one another has produced the angular shafts in the granite which are indicated 

 in figure 1, plate 56. These shafts are bare of fresh debris, their bottoms being 

 well grassed over. 



The pegmatitic character of some of the ridges near the outer portion of the 

 main granitic area about Harney peak, in the Black Hills, is shown on a gigantic 

 scale in the knoll which forms the principal working of the famous Etta tin mine. 

 This is a mass of albite, quartz, and greenish white muscovite in which occur enor- 

 mous crystals of spodumene. The rock carries a small amount of cassiterite and 

 some columbite, and has been called "greisen," incorrectly, by the miners, on ac- 

 count of the presence of the tin ore. The spodumene crystals lie at all angles in 

 the matrix, like so many great sticks of timber, and a few of them are shown in 

 figure 2 of plate 56. One crystal that I measured roughly in the side of one of the 

 old adits was more than 30 feet long and 30 inches wide. The crystals are crossed 

 by numerous fissures, are bounded by imperfect planes, and all seem to lie on edge 

 in the rock. None were observed which had been disturbed by faulting. Many 

 were surrounded by zones of alteration products. The spodumene was thrown 

 on the dump while the property was being worked for tin, but now it is being 

 quarried in a small way for commercial purposes for its lithia content, the cassiterite 

 being thrown to one side. 



In the discussion which followed S. F. Emmons said : 



The granite needles apparently result from the wearing away of the softer schists 

 that once surrounded them. The schists are generally covered by surface accu- 

 mulations and rarely show distinct outcrops. Inclusions of them are, however, 

 found in the central granite mass of Harney peak, and a section is exposed in a 

 road cutting near the hotel. With increased distance from the central Harney 

 Peak mass the granite bodies assume a more distinctly lenticular form, and stand 

 out more and more isolated in the forest covered region where few outcrops of 

 rocks other than granite can be detected. On the outer edges of the area in 

 which granite exposures are found, the granite assumes the form of flat-lying 

 pegmatite veins dipping gently away from the Harney Peak mass. 



