proceedings of petrographic section. 393 



Session of Thursday, December 31 



The Societ} r was called to order at 10.15 a m by Vice-President Edward 

 Orton. 



Professor George H. Barton presented orally a proposition to indorse 

 Lieutenant R. E. Peary's plan of sending scientific parties upon an expe, 

 dition to Greenland during the summer of 1897. 



Remarks in approval were made by Angelo Heilprin, and the matter 

 was then laid upon the table until 2 o'clock p m. 



It was proposed that, in order to save time and secure the presentation 

 of all the papers upon the program within this day, certain designated 

 papers of a petrographic character should be read in a separate section, 

 and it was so voted. 



PROCEEDINGS OF THE PETROGRAPHIC SECTION 



The members interested thereupon proceeded to the laboratory of Dr 

 George P. Merrill, in the National Museum, and at 11.30 a m organized 

 a petrographic section. Professor B. K. Emerson was made chairman 

 and Mr H. W. Turner secretary. 



The proceedings were opened by the reading of a paper entitled — 



APORHYOLITE OF SOUTH MOUNTAIN, PENNSYLVANIA 

 BY FLORENCE BASCOM 



Contents 



Page 



Topographic features of the area 393 



Former and recent investigations 394 



Petrographic structures of the aporhyolite 394 



Comparison with other aporhyolites 395 



Conclusions 396 



Topographic Features of the Area 



The extension of the Blue Ridge in Pennsylvania known as the South mountain 

 lies a little to the east of the middle of the state and stretches in a crescent-shaped 

 curve from the Maryland state line toward the Susquehanna. Twelve miles south- 

 east of Harrisburg it is interrupted by the Susquehanna valley. The same belt of 

 highland reappears at Reading and can be traced for many miles to the northeast, 

 where it merges into the Highlands of New Jersey, which in turn become the 

 Green mountains of Massachusetts and Vermont. 



From the state line to Dillsburg Junction, the most northerly extremity of South 

 mountain proper, is a distance of 40 miles, while the greatest width of the moun- 

 tain is something over 10 miles. South mountain, as the map shows you, is not a 

 continuous mountain range, but rather consists of a series of broken hills and 

 ridges, separated by high plateau-like valleys. The elevations do not exceed 2,000 



