

Watson and Powell — Age of Virginia Piedmont Slates. 39 



occur, grading one into the other. The belt comprises a prob- 

 able average thickness of about one mile of alternating beds of 

 gray to dark gray and black slates, with beds of green and ma- 

 roon slates shown in some sections. Slaty cleavage is promi- 

 nently developed in all the rocks of the belt. Variation is from 

 dense, homogeneous, black graphitic slates of exceeding fine 

 texture, having smooth cleavage surfaces, to rocks of fine gran- 

 ular texture having more or less curved or crumpled cleavage 

 surfaces, and closely resembling phyllites. 



Fig. 5. 



Fig. 5. Granite falls in Occoquan Creek immediately west of Occoquan 

 village, and about }£ mile west of granite-slate contact. 



The black graphitic slates near Dumfries in the Quantico 

 section are reported to contain on analysis 3 per cent of graph- 

 ite. These slates have been exploited for graphite in the 

 Quantico Creek and Potomac Creek sections. In the Potomac 

 Creek section the slates are considerably puckered and twisted. 



The tufaceous character of a part of the beds for a consider- 

 able thickness of many of the sections is apparent to the naked 

 eye. Beds of tufaceous character associated with the slates of 

 the Arvonia district, further south west ward in Buckingham and 

 Fluvanna counties, are even more conspicuously developed in 

 places than in some sections of the Quantico belt. 



The slates proper visually grade northwestward into true 

 phyllites and schists, and are in contact with granites and 

 gneisses which are in part at least of pre-Cambrian age. More 

 orlesspyrite in small and large crystals and in shapeless grains 

 is disseminated through the slates, and frequently forms thin 



