286 A. R. CROOK — MOLYBDENITE AT CROWN POINT, WASHINGTON 



Twelve miles from the head of the lake, on the southwest side, opens 

 up a valley which extends 20 miles westward to near the summit of the 

 Cascades. It discloses folded metamorphic and igneous rocks. Through 

 the valley runs Railroad creek, making a fall of 4,000 to 5,000 feet in 20 

 miles from its source to the surface of the lake, which is only 1,100 feet 

 above tide level. At the head of the valley rises a granite cliff so pre- 

 cipitous that many would-be visitors to the mine withdraw without 

 attempting the ascent. 



That the molybdenite-bearing ledge was ever discovered is surprising, 

 and gives an indication of the minute scrutiny with w T hich prospectors 

 have gone over the country. The first tunnel is 900 feet above the 

 miners' cabin at the foot of the cliff, and the face of the cliff makes an 

 angle of from 60 to 80 degrees from the horizontal. Access to the mine 

 is made possible by a rope fastened to an iron peg in the rocks (plate 12, 

 figure 1). 



Form of the Deposit 



The molybdenite occurs in a quartz vein, a blanket vein outcropping 

 along the nearly perpendicular face of a granite cliff for several hundred 

 feet. In general, it is nearly horizontal, but at times has an inclination 

 of from 5 to 6 degrees toward the west (plate 12, figure 2). Its average 

 thickness, which is from 2 to 3 feet, and the general horizontal position 

 are well shown in plate 13, figure 1). Two tunnels have been driven 

 into the cliff, one extending 195 feet toward the northeast and the other 

 80 feet westerly. There are something more than 100 feet of open work- 

 ings. The molybdenite does not occur near the center of the vein. Thus 

 it differs from the Mono county, California, occurrence* At Crown 

 Point it is found in small seams several inches in thickness, extending 

 in all directions through the quartz vein from side to side. At no time 

 is the molybdenite found in the accompanying granite, but is always 

 separated from it by a layer of quartz. 



Inclosing Rock 



The rock in which the molybdenite-bearing quartz vein is found is a 

 greenish gray biotite granite. It is from medium to fine grained, and 

 even in texture. Of the composing minerals, the quartz is firm, com- 

 pact to granular, with occasional crystal faces, and usually glassy and 

 white when not discolored by iron oxides. The feldspars are opaque 

 and colored greenish by the decaying biotite, which is changing to 

 chlorite. Thin-sections show under the microscope that the quartz is 

 well filled with fluid inclosures and is xenomorphic in relation to the 

 feldspar and biotite. The feldspars are so kaolinized as to be almost 



