128 F. W. Clarke — New Occurrence of Gyrolite. 



Art. XV. — A New Occurrence of Gyrolite • by F. W. 



Clarke. 



In the autumn of 1888, during a visit to the New Almaden 

 Quicksilver Mine in California, Dr. D. T. Day of the United 

 States Geological Survey was shown specimens of a mineral 

 which was locally supposed to be white fluor spar. It occurred 

 in well developed crystals lining crevice veins in the mine, and 

 was easily recognizable as apophyllite. Dr. Day secured a 

 good series of the specimens, and finally turned them over to 

 me for examination. The largest crystals were about two 

 centimeters in diameter, and fairly transparent, and grew out 

 of crystalline masses of considerable thickness ; the exposed 

 definite faces nearly meeting at the center of the seam or vein. 

 All of the specimens were saturated with bituminous matter, 

 but except for that staining they were quite colorless. 



In several of the specimens received the wall of the seam 

 was distinctly shown, and between it and the crystalline apo- 

 phyllite there was a fibrous layer from one to three centi- 

 meters in thickness. That layer was also colorless, except for 

 bituminous staining, and on account of its relations to the 

 apophyllite it appeared to deserve investigation. A sufficient 

 quantity of the material having been selected, it was digested 

 for about twenty-four hours with ether in order to cleanse it 

 from bitumen, and then analyzed. I give the result in com- 

 parison with How's figures for a Nova Scotia gyrolite. 



Clarke. How. 



H 2 14-60 15-05 



Si0 2 52-54 51-90 



A1 2 0,[ 7 1-27 



CaO 29-97 29-95 



MgO 0-08 



K 2 1-56 1-60 



Na 2 0-27 



F 0-65 



10030 99-85 



Less 0-27 



100-03 



It will at once be seen that the two analyses, except for the 

 small amount of fluorine, coincide quite sharply, and establish 

 the New Almaden mineral as gyrolite. The Nova Scotian 

 gyrolite is also associated with apophyllite, from which 

 species How supposed it to be derived.* In the present 

 instance, however, the relative position of the two minerals in 

 the vein suggests that the gyrolite is the older ; and that the 



* See this Journal, II, xxxii, 13. 



