352 University of California Publications in Geology [Vol. 10 



An analysis of this brown variety gave : 



Si0 2 44.10% 



A1 2 3 24.20 



CaO 25.20 

 H 2 5.86 



99.36 



Laumontite. — The green prehnite has laumontite coating it in small 

 divergent columnar and fibrous masses. It has the appearance of a 

 secondary alteration of the prehnite and in all probability it is such, 

 although there is no direct evidence to prove it. The mineral is snow- 

 white with vitreous luster. Some of the stouter needles have the 

 characteristic oblique cleavage ends. It fuses readily to a vesicular 

 glass and is easily and completely soluble in HC1, yielding a thick 

 gelatinous mass. 



An analysis gave : 



Si0 2 53.49% 

 A1,0 3 22.01 

 CaO 10.80 

 MgO tr. 

 H 2 13.39 



99.69 



Opal. — White hyalite coats the walls of some of the cavities in the 

 feldspathic pegmatite, and it coats the apophyllite and okenite to some 

 extent. Under the electric spark it shows strong yellow luminescence, 

 which is an indication that it contains uranium. The amount of 

 material on hand is too small to determine the uranium chemically. 



Besides this hyalite opal there is considerable wood opal in some 

 portions of Sky Blue Hill. It is dark brown and black, becoming gray 

 by heating. It shows the wood structure and some of it has small 

 calcite rhombohedrons deposited on it. 



SULPHIDE MINEEALS 



Arsenopyrite, pyrite, galena, sphalerite, greenockite, bornite, 

 chalcopyrite, and tetrahedrite occur in the Commercial Kock quarry, 

 mostly in association with the pegmatite intrusives. 



Arsenopyrite has been found in a few small crystals about two 

 millimeters long. 



Pyrite is more disseminated than the other sulphides. It occurs in 

 grains, cubes and pyritohedrons, and much of it is altered into brown 

 limonite pseudomorphs. 



