Ancient Rhy elite of South Jfotmtain, Pennsylvania. 5 1 



growth of epidote had continued, even after the available man- 

 ganese supply had become exhausted. 



The deeply colored, typical piedmontite is irregularly dis- 

 tributed as a microscopic constituent, being most abundant in 

 the reddest rhyolites, and in them, most developed in the feld- 

 spar phenocrysts, where it is evidently secondary. At some 

 points a considerable thickness of radiating piedmontite needles 

 occupy cavities and seams in this rock. It is the occurrence 

 of such macroscopic masses of the pure mineral which makes 

 this region an important piedmontite locality ; for, while this 

 substance is becoming constantly more widely known as a 

 microscopic rock constituent, the places where it occurs in 

 specimens of macroscopic size and beauty are still very few. 

 In one case piedmontite was found replacing old spherulites 

 and projecting from the sides of ovoid cavities into a mass of 

 enclosing scheelite. 



These three types of occurrence will now be described in the 

 inverse order of that in which they have just been mentioned. 



1. Minute crystals in scheelite. — In the area of rhyolite 

 breccia mentioned in the writer's former paper as occurring in 

 the Buchanan valley, two miles north of the Chambersburg 

 turnpike,* a large block of the acid lava was found behind 

 Musser's store, in which fairly well crystallized piedmontite 

 was abundant. The manganese epidote here occurred wholly 

 or partly filling ovoid areas which resemble old spherulites. 

 The rock itself is banded with lines of flow-structure and has a 

 deep red color from the piedmontite disseminated through it. 



Associated minerals are quartz and rarely hematite, but the 

 center of the cavity is usually occupied by a white vitreous 

 mineral, into which the terminations of the minute piedmon- 

 tite needles project, and which, quite contrary to expectation, 

 proved to be scheelite. 



The projecting ends of these radiating needles, in spite of 

 their minute size, offer the best material yet found for the 

 study of the crystallographic and optical properties of the 

 South Mountain piedmontite. When a little of the white friable 

 matrix is gently pulverized and mounted in balsam for the 

 microscope a very beautiful result is secured. An abundance 

 of sharply defined crystals are seen bounded by the character- 

 istic planes of epidote, but on account of their intense pleo- 

 chroism exhibiting, when viewed with one Nicol, many 

 tints of yellow, orange, carmine and amethyst. These little 

 crystals, although very sharp and brilliant under the micro- 

 scope, are rarely over 0'2 mm in length and 0'05 mm in breadth. 

 They are therefore too small to measure, although some of 



*Loc. cit. p. 492, and map on p. 483. 



