396 PROCEEDINGS OF WASHINGTON MEETING. 



Many specimens showing lithophysae were collected. The lithophysse range from 

 one-eighth of an inch to an inch in diameter and crowd every spare inch of the rock 

 surface. Sometimes the concentric spaces between the petals of the lithophysse are 

 filled with quartz and hematite, but often they are invested with beautiful radiat- 

 ing crystals of piedmontite. 



In other localities the aporhyolites are more or less charged with piedmontite, 

 but its occurrence in lithophysse has not before been observed. In the thin-section 

 a markedly concentric arrangement of structures is to be noted. The center of a 

 lithophysse is a quartz-feldspar mosaic similar to the crystallization of the ground- 

 mass and may contain phenocrysts of quartz or feldspar. This center is surrounded 

 by a border of brilliant radiating crystals of piedmontite and granular quartz. Pro- 

 jecting into this area of secondary material and resting upon the inner wall of a 

 zone formed of a quartz-feldspar mosaic are traces of crystals showing prismatic 

 and pyramidal faces. The inner wall is studded with these crystals, which in most 

 cases are only preserved in outline. In other cases the quartz or alkali feldspar 

 substance is retained. 



Surrounding the lithophysae and separating it from the groundmass is a fourth 

 zone showing spherulitic and micropoikilitic crystallization. These zones are not 

 fixed in number and may be many or few. The radiating crystallization may be 

 completely or partially replaced by the granular. The iriterlamellar or interzonal 

 spaces are alike filled with quartz and piedmontite. 



Conclusions 



Such are the most instructive features of the material recently procured. They 

 serve to further emphasize the characters which have already been described * as 

 peculiar to the aporhyolite, namely, the occurrence of fluxion, spherulitic, litho- 

 physal, and perlitic structures associated with a holocrystalline groundmass and 

 indicating by their pr-essure the secondary character of that groundmass. 



This material further substantiates the claim made for the secondary origin of 

 the micropoikilitic structure in the aporhyolite of South mountain. 



Finally, the material discloses a new occurrence for piedmontite. 



It is hoped eventually to map all of the South Mountain region in detail. A 

 preliminary survey of the area leads one to expect no new rock types and no apor- 

 hyolites showing original structures in a more perfect state of preservation than 

 does the material which has just now been discussed. 



The paper was illustrated by a map of South mountain and a suite of 

 specimens showing the typical structures of the mineral. 



It was discussed by A. C. Lane, who corroborated the interpretation 

 of the micropoikilitic structure as secondary in acid volcanics in the Lake 

 Superior region. J. P. Iddings remarked that the micropoikilitic struct- 

 ure was undoubtedly both a secondary and a primary structure. While 

 its secondary character in the case of the aporhyolite seemed beyond 

 question, its primary origin in some porphyrites was equally certain. 

 Whitman Cross corroborated J. P. Iddings. 



Bulletin U. S. Geological Survey, no. 14. 



