394 PKOCEEDINGS OF WASHINGTON MEETING. 



feet. Jacks mountain, Rocky ridge, Green ridge, and Piney mountain are some of 

 the more prominent highlands. 



Former and recent Investigations 



Throughout the South Mountain district are abundantly exposed distinct and 

 interesting rock types, whose character has already been announced.* In 1892 

 detailed mapping was accomplished in the southern portion of the district. Field- 

 work was resumed for a brief season during the fall of 1896. At this time further 

 illustrations of the structures peculiar to aporhyolites were secured, and these it is 

 proposed to present to you today. 



Petrographtc Structures of the Aporhyolites 



Rocky Ridge and Raccoon creeks, eroding the aporhyolites for a distance of five 

 miles respectively, furnish a collecting ground for acid volcanic material unsur- 

 passed in any other portion of South mountain. The highland forming the water- 

 shed between these two creeks supplies to the streams brilliantly colored specimens, 

 showing lines of flow of peculiar delicacy and intricacy. The crumpling of a fluid 

 layer moving in a more viscous layer and retarded by it is conspicuously observable 

 in some specimens. 



There also occur here compact bluish gray aporhyolites studded with spherulites 

 of a uniform size (three-eighths of an inch in diameter) and a deep blue color. The 

 rock is porphyritic, and the feldspar phenocrysts are scattered indiscriminately 

 through spherulites and matrix, which are not easily separated mechanically. In 

 the thin-section the spherulites show, under crossed nicols, the micropoikilitic 

 structure, which merges into a narrow border of a granular quartz- feldspar mosaic, 

 so similar to the crystallization of the groundmass as only to be distinguished by 

 the rim of iron oxide separating spherulite and groundmass. Occasionally the 

 spherulites are vesicular and bear tridymite spherulites on the walls of the vesicles, 

 which are filled with quartz. The groundmass usually shows perlitic parting, ob- 

 literated under crossed nicols by a granular crystallization. 



At approximately the same locality were found aporhyolites, in which the 

 spherulites are from one-half to three-fourths of an inch in diameter and drop out 

 very readily from the matrix in which they are imbedded. In this case the micro- 

 scope shows no border of granular crystallization surrounding the spherulites. The 

 groundmass and the spherulites are sharply separated by the presence in the latter 

 of a pronounced micropoikilitic structure of a rather coarse grain. This was true 

 of all the spherulites which were sectioned. Such occurrences as these of the micro- 

 poikilitic structure make it impossible to escape the conviction that this structure 

 may be a secondary one, although it has been otherwise interpreted by Dr Clem- 

 ents in similar acid volcanics of the Michigamme district. In the South mountain 

 aporhyolites the structure has occurred either as a replacement of a radiating 

 crystallization, where it must be subsequent to the consolidation of the lava, or in 

 a presumably devitrified groundmass, where it must also be subsequent to consoli- 

 dation. 



This structure is also found in some of the quartz-porphyries of South mountain, 

 where its secondary origin is not so clearly proven. The occasional character of 

 its occurrence, however, and the close association of the quartz-porphyries possess- 



*G. H. Williams : Am. Jour. Sci , vol. xliv, 1892, pp. 482-496. Bulletin U. S. Geological Survey, 

 no. 136. 



