832 THE JOURNAL OF GEOLOGY. 



The subject discussed in this paper forms a part of a thesis, 



on South Mountain, presented at the Johns Hopkins University. 



The petrographical study was conducted in the petrographical 



laboratory of that institution, under the immediate' supervision 



of Professor G. H. Williams, to whose valuable suggestions and 



stimulating interest the writer is in every way indebted. 



F. Bascom. 



Petrographical Department, 

 State University, Columbus, Ohio. 



