40 IVatson and Powell — Age of Virginia Piedmont Slates. 



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films or veinlets in the cleavage. In a 

 number of places the pyrite in the 

 slates is sufficiently concentrated to 

 encourage mining. Much quartz is 

 present in the slates in places, occur- 

 ring chiefly as eyes and stringers, 

 sometimes as veins, which may coin- 

 cide in direction with that of cleavage 

 or may cut across it. 



In several of the measured sections, 

 Marumsco and Powells creeks in the 

 Quantico slate belt, and south of James 

 River in the Arvonia slate belt, a light 

 gray, hard and dense-textured rock, 

 which has been identified microscop- 

 ically as an altered (metamorphosed) 

 rhyolite, occurs interbedded with the 

 slates (fig. 6). Thin sections of the 

 rock show the following minerals : 

 Quartz, sericite, biotite, chlorite, mag- 

 netite, tourmaline, apatite, zircon, and 

 yellow glass. The exact thickness of 

 this rock in the Quantico belt could 

 not be ascertained because of lack of 

 exposures, but in the Arvonia belt it 

 is about 18 feet. It is strikingly uni- 

 form in lithology at every point ob- 

 served. Except for small parallel 

 elongated yellow specks and areas, 

 which are identified as feldspathic(?) 

 glass under the microscope, the fresh 

 rock appears more massive than schis- 

 tose, but partially weathered speci- 

 mens of the rock invariably show 

 pronounced schistosity. The longer 

 axis of the yellow glass areas and the 

 direction of schistosity are oriented in 

 the same plane. 



Occasional dikes of both basic and 

 acid igneous rocks are intruded in the 

 slates, as observed in the Occoquan 

 and Quantico creeks sections (fig. 7). 



Fossils. — During the summer of 

 1909, while studying the Powells 

 Creek section, 3 miles north of Dum- 

 fries in Prince William County, the 

 junior author found fossils in the 

 smooth black graphitic slates, exposed 



