Watson and Powell — Age of Virginia Piedmont Slates. 35 



conglomerates, and crystalline limestones. These are metamor- 

 phosed and intruded by dikes and masses of diabase, diorite, 

 and gabbro. Over parts of the eastern and central portions of 

 the region are areas of altered volcano-sedimentary rocks, which 

 extend southward into North Carolina. 



Slate Areas of the Crystalline Region. 



There are five principal slate areas occurring within the 

 limits of the crystalline region of Virginia. These are shown 

 on the accompanying map, figure 1. Named in the order of 

 their present importance, the areas are : (1) the Arvonia belt 

 in Buckingham and Fluvanna counties ; (2) the Keswick- 

 Esmont belt in Albemarle County ; (3) the Snowden belt in 

 Amherst and Bedford counties ; (4) the Warrenton belt in 

 Fauquier and Cnlpeper counties; and (5) the Qnantico belt 

 in Prince William, Stafford, and Spottsylvania counties. 



The discovery of fossils in three of the areas (Snowden, 

 Arvonia, and Qnantico) has definitely determined their age — 

 the Snowden area as Cambrian, and the Arvonia and Qnantico 

 areas as Ordovician. Fossils have not been found in either of 

 the other two areas, Warrenton and Keswick-Esmont, but the 

 relations of the slates to associated rocks of known age in or 

 near these areas fix the age of the slates beyond reasonable 

 doubt as Cambrian. 



The Snowden slate area, flanking the southeast slope of the 

 Blue Ridge in Amherst and Bedford counties, includes a series 

 of beds of conglomerates, sandstones, and slate resting uncon- 

 formably on a basement of igneous rocks. The sedimentary 

 beds are in a highly metamorphosed condition, and were 

 regarded by Rogers as Huronian, and by J. L. and H. D. 

 Campbell as pre-Cambrian, but the discovery of fossils in them 

 has definitely determined their age as Cambrian.* The sand- 

 stones of this series contain fossil borings of Scolithus linearis, 

 which with the other data marks them as Cambrian in age. 

 No fossils have been found in the slates of this series, but the 

 Scolithus sandstone is observed to dip under the slates, which 

 indicates that the latter cannot be older than Cambrian. f 



The Warrenton slate area in Fauquier and Culpeper counties 

 comprises a sedimentary series of rocks of Cambrian age 

 (Loudoun) in close association with a pre-Cambrian (Algon- 

 kian) series of basic volcanic rocks (Catoctin schist) and their 

 equivalent pyroclastics (tuffs). The sedimentary series (Lou- 

 doun formation) includes coarse arkoses, sandstones, quartzites, 



* Pre-Cambrian Geology of North America; Bull. No. 360, U. S. Geol. 

 Survey, 1909, p. 675. flbid., p. 694. 



