482 G. H. Williams — Volcanic Bocks of South Mountain 



varying very much in the character and size of the coarser 

 material. The feldspathic character of these shales and sand- 

 stones is very distinctly marked beneath the Scolithus quartz- 

 ite, both in the Balcony Falls and the Monterey sections. 



If these two sections are compared with that at " Chiques 

 Rock" and south to Columbia, in Lancaster county, Pa., it 

 will be at once observed that the Scolithus qnartzite, while 

 the highest band of quartzite in the Balcony Falls and the 

 Monterey sections, is the lowest in the " Chiques Rock " sec- 

 tion which has the lower feldspathic sandstone and shales 

 apparently above the Scolithus qnartzite. It is from this fact 

 that it is stated, in the first part of this paper, that the feld- 

 spathic sandstones and shales were thrust over on the Scolithus 

 sandrock in the " Chiques Rocks" section. 



Art. LXV. — The Volcanic Rocks of South Mountain in 

 Pennsylvania and Maryland • by George H. Williams. 

 With Plate X* 



Contents. — 1. Object of this paper. 2. Supposed sedimentary origin of the 

 South Mountain volcanic rocks. 3. Petrographical character : a) The acid rocks 

 — rhyolites ; b) The basic rocks — basalts ; c) The pyroclastic deposits — tuffs and 

 breccias. 4. Geological occurrence and relations to the sandstone. 5. Chemical 

 alteration and metamorphism. 6. Comparison with other regions. 



[Read before the National Academy of Sciences, Nov. 2, 1892.] 



1. Object of this paper. — It is the object of the present 

 communication to announce the identification of an extensive 

 area of very ancient volcanic rocks which compose an im- 

 portant part of the South Mountain, south of the Susquehanna 

 River. The brief preliminary description of these rocks, 

 which is all that can now be attempted, will, it is hoped, suf- 

 fice to show that the hitherto accepted theory of their sedi- 

 mentary origin has been based on a misinterpretation of the 

 facts which they exhibit. 



The rocks in question preserve abundant and convincing 

 evidence — both structural, chemical, and petrographical — of 

 their original character and genesis. At the same time they 

 show various phases of alteration by recrystallization and 

 dynamic agencies which render them valuable for the study of 

 many problems of metamorphism. 



* The writer is under great obligations for many of the facts contained in this 

 paper to Miss Florence Bascom, who has mapped in great detail one of the most 

 diversified portions of South Mountain, near Monterey, Pa. She has collected a 

 large material upon which she is now at work at the Johns Hopkins University, 

 and her results, to be published in due time as a thesis, will go far toward fur- 

 nishing detailed proof of the general conclusions here set forth. 



