Watson and Powell — Age of Virginia Piedmont Slates. 43 



tions of the slates are the same. Moreover, the slates in the 

 two belts are alike in composition in being both graphitic and 

 magnetitic. 



There is in Louisa County and a part of Spottsylvania an 

 area between the two belts, representing a distance of about 35 

 miles in direction of strike, in which the very dark gray to 

 black graphitic slates have not been observed. While the true 

 slates of this lithologic type have not been found in the small 

 area between the two belts, the phyllites and schists, into which 

 the slates of the Qnantico belt grade, are traced continuously 

 from the Qnantico belt to the Arvonia belt, without change 

 of structural relations indicated. It seems most likely there- 

 fore that the basin in which terrigenous and pyroclastic sedi- 

 ments were deposited during Ordovician times was a continuous 

 one, and, so far as traced in Virginia, extended from the south- 

 ern part of Buckingham County in a northeasterly direction to 

 within 10 miles southeast of Alexandria, where the rocks pass 

 beneath the Coastal Plain sediments of Cretaceous and Ter- 

 tiary age. 



The slates of these two belts, Qnantico and Arvonia, in which 

 upper Ordovician fossils have been found, represent the east- 

 ernmost extension in Virginia of sedimentation during Ordo- 

 vician time. West of the Blue Bidge the extensive areas of 

 Martinsburg shale were laid down about the same time and 

 are regarded as the probable equivalent of the Piedmont slates 

 of the Qnantico and Arvonia belts. Differential metamorph- 

 ism has emphasized a fundamental difference in the eastern 

 Piedmont slates and their equivalent Martinsburg shale, west 

 of the Blue Ridge. 



Arvonia Slate Belt. 



In 1892,* Mr. N. H. Darton announced the discovery of 

 organic remains in the rooting slate at Arvonia, Buckingham 

 County, Virginia. The slabs collected by Mr. Darton were 

 submitted to Dr. Walcott, who made the following statement 

 regarding them :f "I have studied the specimens of slate show- 

 ing crinoidal remains and come to the conclusion that they 

 belong to the Trenton-Lorraine or upper portion of the Ordo- 

 vician fauna. One of the large columns is closely allied to 

 tSchizocrinus vwdosus, and some of the heads, although indis- 

 tinct, approach closely to Heterocrinus and Poterocrinus. If 

 these suggestions are correct, the slates are to be correlated 

 with Lorraine or Hudson series and in the same horizon with 

 the Peach Bottom slates of Pennsylvania." 



* Darton, N. H. Fossils in the "Archaean" Rocks of Central Piedmont, 

 Virginia. This Journal, vol. xliv, pp. 50, 52. flbid., p. 52. 



