322 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



The Conshohocken diabase therefore falls into class II, order 4, rang 4, 

 subrang 3. This indicates close similarity to gabbro, differing chiefl}' in 

 the relation of the feldspar of the other salic constituents. 



Feldspar is dominant over quartz, but not to an extreme degree. The 

 anorthite molecule is dominant over the alkali feldspars and soda is the 

 dominant alkali. Augite is a critical mineral, the structure is mego- 

 phitic, and the name augite-ophito-bandose. 



The diabase of Williamson's point falls into class III, order 5, rang 4, 

 subrang 3. It is an augite-ophitic-auvergnose. This means that the 

 salic and femic constituents are present in about equal amounts; that 

 of the salic constituents feldspar predominates ; that among the feldspars 

 the anorthite molecule dominates, while of the alkalies soda dominates. 

 Augite is an abnormative mineral and the structure is micro-ophitic. 



PEQMA TJTES 



In the southwestern portion of the belt there occur sporadically 

 numerous pegmatite dikes which have considerable width, can be 

 traced for long distances, and possess a commercial value. 



Where they are favorably situated with reference to drainage lines the 

 acid feldspar in which they abound is completely kaolihized and fur- 

 nishes large kaolin plants with clay. Elsewhere the fresh feldspar is 

 utilized for dental purposes and for pottery. 



Geologic History of the Piedmont District of Pennsylvania 



The sedimentary series of the Pennsylvania Piedmont indicate that 

 a comparatively limited land mass existed on the eastern border of the 

 continental plateau in late pre-Cambrian time ; that during Cambro- 

 Ordovician time there were deposited in an encroaching sea upon the 

 western border of this land mass arenaceous, calcareous, and argillaceous 

 sediments successively. There is no record in the Pennsylvania Pied- 

 mont of further sedimentation during Paleozoic time. It is not probable, 

 however, that the subsequent land movements immediately followed on 

 the deposits of argillaceous material, and that sedimentation in this 

 district closed. 



The amount of material deposited in Paleozoic time, which has dis- 

 appeared through subsequent erosion, is not known, but was presumably 

 of considerable extent. Beginning, perhaps, at the close of Ordovician 

 time and continuing until the close of Paleozoic time, there were earth 

 movements along the Atlantic border which resulted in the uplifting, 

 folding, and recrystallization of these sediments. 



