490 G. H. Williams — Volcanic Hocks of South Mountain 



run) has any ferro-magnesian constituent as yet been detected. 

 In this case biotite is abundant as an original mineral. The 

 ground mass of the rhyolites is for the most part a quartz- 

 feldspar mosaic of varying grain, much of which is the result 

 of devitrification and recrystallization, though some of it is 

 also probably original microgranite. 



Fig. 6. — Axiolitic and vesicular flow 

 structures. No. 77 from near Snowy 

 Mtn., mag. 5 diameters. 



Pig. 7. — Flow structure. No. 61, 

 South of Willow Grove, Pa., mag. 5 

 diameters. 



Attention has been called by Tyson,* Huntf and Frazer^: 

 to the beauty of the South Mountain porphyries, and to the 

 fact that they are so susceptible of a high polish as to make 

 them valuable for decorative purposes. Many of them closely 

 resemble the famous porfido rosso antico of Egypt, which is 

 largely used by the lapidaries of Rome. 



b) The basic rocks, basalts. — The basic lavas of South 

 Mountain occupy an area about one-half as large as that cov- 

 ered by the acid ones. They reach their maximum develop- 

 ment near the State line, where, along the southern edge of 

 Pennsylvania and for a considerable distance into Maryland, 

 they form the entire width of the volcanic belt. North and 

 south of this main body, the basic rocks or greenstones are 

 everywhere met with as narrow bands intersecting the rhy- 

 olites and following the general trend of the mountain. These 

 bands differ much in their width, but seem to be most devel- 

 oped along the eastern flanks of Green Ridge and Piney 

 Mountain. 



* First Annual Eeport, 1860. Appendix, p. 3. 



f Proc. Am. Assoc. Adv. Science, 1876, p. 212 (compares them with the Elf- 

 da] en porphyries). 



% Second G-eol. Survey of Penn., vol. CC, p. 285. 



