50 G. H. Williams — Piedrnontite and Scheelite from the 



Neither of the processes described presents in general 

 advantages equal to those of the manganous chloride method 

 to which reference has been made, but under special conditions 

 the j may be found useful. 



In concluding I would express my thanks to Professor 

 Gooch for valuable advice and assistance given during the 

 course of the investigation. 



Akt. VII. — Piedrnontite and Scheelite from the Ancient 

 Rhyolite of South Mountain, Pennsylvania ; by Geokge 

 H. Williams. 



[Published with permission of the Director of the U. S. Geological Survey.] 



In a recent paper on the Ancient Volcanic Rocks of South 

 Mountain in Pennsylvania and Maryland, the identification of 

 the manganese epidote, piedrnontite, in the acid lavas was 

 incidentally mentioned.* Several points of interest regarding 

 the occurrence, association and chemical composition of this 

 mineral, which deserve a special notice, have been brought out 

 by its subsequent study. 



The acid or rhyolite lavas so extensively developed in South 

 Mountain possess a wide range of color, within which varying 

 shades of pink, maroon and purple are conspicuous. These 

 reddish varieties show many signs of being rich in manganese. 

 Aside from their color, clefts in the rock are often stained 

 black by the oxides of this element, while ordinary weathering 

 not infrequently brings about a similar result. In the basic 

 and more highly ferruginous lavas, conditions favorable to the 

 formation of epidote have at some time obtained to an extent 

 rarely surpassed. Amygdules are filled with this mineral, while 

 in some localities the rock itself is fairly changed to a mass of 

 epidote. Throughout their entire extent the basic lavas have 

 been " epidotized " to a remarkable degree. The tendency to 

 epidote formation is likewise apparent in the acid rocks, espe- 

 cially when they are examined under the microscope. Accord- 

 ing to the supply of manganese present, however, the common 

 epidote shows every graduation from the faintest rose tinge, 

 through the more pronounced pinks of withamite, to the rich 

 deep carmine of typical piedrnontite. All of these varieties fre- 

 quently occur within the limits of a single thin section, or even 

 as an irregular coloring of a single crystal or group of crystals. 

 In such cases, the deepest color is at the center, as though the 



* This Journal, III, vol. xliv, p. 495, Dec. 1892. 



