Ancient Rhyolite of South Mountain, Pennsylvania. 53 



The determination of the colorless mineral associated with 

 the piedmontite at this locality as scheelite was so unexpected 

 that a few words may be devoted to its properties. It is quite 

 brittle and has a vitreous to resinous luster and fairly good 

 pyramidal cleavage. No trace of crystal form was observed, 

 but the cleavage angle measured approximately 100°. In con- 

 verged polarized light the pyramidal cleavage flakes show a 

 uniaxial figure of positive character and strong double refrac- 

 tion. Moistened with hydrochloric acid the fragments became 

 yellow and then, as the solution dries it becomes a deep blue. 

 Fused with Ka 2 C0 3 on charcoal and dissolved in water the solu- 

 tion is a deep blue. The phosphor-salt bead is green when hot 

 in the reduction flame, but it remains a deep emerald tint on 

 becoming cold, instead of turning blue as the scheelite bead 

 usually does. The specific gravity determined by a pycnom- 

 eter with the little material at hand gave 5*61, which, though 

 somewhat low for scheelite, is as good a result as could be 

 expected with the somewhat porous substance. 



2. Radiating aggregates filling veins in the rhyolite. — The 

 locality which has furnished the greatest abundance of South 

 Mountain piedmontite is on the west flank of Pine Mountain 

 about one mile north of Monterey station. The mineral has 

 here a deep carmine color and forms spherulitic aggregates of 

 needles upon which no trace of crystal form is visible. They 

 are crowded closely together and fill irregular veins which 

 vary in width from 5 or 6 cm to the finest possible threads. The 

 average diameter of the radiating aggregates is from 5 to 7 mm , 

 although occasional needles have been observed measuring as 

 much as 10 mm in length. 



In microscopic section these aggregates present a brilliant 

 appearance. The piedmontite needles have the same optical 

 properties as the more perfect crystals already described ; but, 

 since there is no parallelism of their a and c axes, the fibers 

 all show an amethystine color in the direction of their elonga- 

 tion, but various shades of carmine, yellow and orange in 

 transverse directions. Some of the needles show in their 

 pleochroism a complex twinning structure like that figured by 

 Laspeyres.* The microscope shows further that the needles 

 of these radiating aggregates are minutely intergrown with 

 clear limpid quartz. This mixture is so extremely intimate as 

 to render the obtaining of pure material for analysis very diffi- 

 cult. A powder obtained with the Thoulet solution was care- 

 fully analyzed by Mr. W. F. Hillebrand, of the U. S. Geolog- 

 ical Survey, with the following result : 



*Zeitschrift fur Kryst, vol. iv, PI. XT, fig. 8, 1880. 



