Watson and Powell — Age of Virginia Piedmont Slates. 33 



Art. IV. — Fossil Evidence of the Age of the Virginia 

 Piedmont Slates / by Thomas L. Watson aud S. L. 

 Powell. 



Contents: 

 Introduction. 



Virginia Piedmont Province. 

 Slate areas of the crystalline region. 

 Quantico slate belt. 



Fossils. 

 Arvonia slate belt. 



Introduction . 



Recent detailed field study of the slate areas in the crystal- 

 line (Piedmont) region of Virginia by the State Geological 

 Survey has resulted in much important information bearing 

 on the lithologic characters, structural and age relations of the 

 rocks, and on the sulphide ore-bodies (veins) associated with 

 the slates of the northeastern belt. Of especial interest are : 



(1) discovery of fossils in the easternmost one of the slate areas ; 



(2) recognition of volcano-sedimentary beds intimately associ- 

 ated with the slates in several of the areas; and (3) evidence of 

 the age relations of a part at least of the sulphide veins in the 

 northeastern portion of the crystalline region, which hitherto 

 have been assumed to be pre-Cambrian. 



The present paper treats only of the discovery of fossils in 

 the Quantico slate belt, with a brief statement of its strati- 

 graphic position, and of that of the other slate belts in the Vir- 

 ginia crystalline region. Discussion of the age relations of the 

 sulphide veins and of the volcano-sedimentary beds associated 

 with the slates will be treated in another paper, now in pre- 

 paration. 



Virginia Piedmont Province. 



The Virginia Piedmont province (crystalline area) lies be- 

 tween the Coastal Plain and the Appalachian Mountains. It 

 extends from the Blue Ridge eastward to the western margin 

 of the Coastal Plain, and it widens southward (map, fig. 1). 

 Its width increases from 40 miles in the northern portion 

 along the Potomac River to nearly 175 miles along the Virginia- 

 Carolina boundary. The rocks of this region are the oldest in 

 the state, and, excepting the areas of Triassic rocks, they are 

 all crystalline. They comprise both igneous and sedimentary 

 masses, in many places so altered from metamorphism, chiefly 

 pressure and recrystallization, that their original character is 

 indistinguishable. 



The region is made up of a complex of schists, gneisses, and 

 granites, with which are associated some slates, quartzites and 



Am. Jour. Sci.— Fourth Series, Vol. XXXI, No. 181.— January, 1911. 



