Ancient Mhyolite of South Mountain, Pennsylvania. 55 



This analysis is of especial interest in showing that the South 

 Mountain pieduiontite is a connecting link between three 

 recognized members of the epidote group. In the first place, 

 the amount of cerium and other rare earths which it contains 

 indicates a considerable admixture of the allanite or orthite 

 molecule ; while the presence of both manganous and man- 

 ganic oxides gives this mineral a position intermediate between 

 true pieduiontite and the mangan-epidote of Flink. Copper 

 occurs extensively in the rocks which carry the piedmontite. 



Another locality for piedmontite filling veins in the rhyolite 

 was found near the head of Miney Kun below the Clermont 

 Hotel. While similar to that on Pine Mountain, the mineral is 

 here less abundant. 



3. Piedmontite as a microscopic constituent of the rhyolite. 

 — Although piedmontite has as yet been observed in macro- 

 scopic quantities in the South Mountain rhyolite only at the 

 three localities above mentioned, it is quite generally dissemi- 

 nated as a microscopic constituent. Other occurrences like 

 those already described will doubtless come to light as the 

 detailed examination of the rhyolite area proceeds. 



The reddest rhyolites contain microscopic piedmontite most 

 abundantly and this color, as in the case of the well-known 

 u porfido rosso antico," seems to be due to the mineral. The 

 most representative specimens of this variety are from the 

 small area of rather coarse and porphyritic rhyolite occurring 

 just at Monterey station, Pa. This rock, which may be re- 

 garded as a type of all the others of this region like it, has a 

 micropoikilitic* groundmass enclosing good-sized phenocrysts 

 of checkered anorthoclase and smaller ones of rounded quartz. 

 It shows no particular evidence of dynamic action, though 

 there is some secondary quartz and sericite jDresent. The 

 groundmass is full of black specks, probably manganese oxide 

 resulting from the alteration of piedmontite, while there is 

 every indication that the latter mineral, along with its asso- 

 ciated rose and common epidote, is itself always of secondary 

 origin. It occurs in veins or cracks in the feldspar phenocrysts 

 and groundmass or in isolated patches, usually in association 

 with secondary quartz x epidote and black manganese oxide. 

 The piedmontite is not infrequently surrounded by a zone of 

 pale rose or colorless epidote, which, however, seems to be 

 rather granular than of continuous crystalline structure with 

 the deep red mineral in the center (fig. 2). 



As the microscopic piedmontite agrees in all its physical 

 properties with the macroscopic, except for its greater ten- 

 dency to mix in all proportions with the epidote molecule, no 



* G. H. Williams: ''Ou the use of the terms poikilitic and micropoikilitic in 

 petrography;" Journal of Geology, vol. i. p. 176, 1893. 



