Watson — Dike of Diabase in the Potsdam Sandstone. 89 



Art. VIII. — On a Dike of Diabase in the Potsdam Sand- 

 stone in the Valley of Virginia / by Thomas Leonard 

 Watson. 



The occurrence of igneous rocks in the Paleozoic sediments 

 of the Appalachian region is unusual. In 1883 Professor 

 Fontaine* made record of basic igneous material intersecting 

 the Virginia Appalachian Paleozoic sediments. This record 

 was of a dike of igneous rock penetrating the Valley (She- 

 nandoah) limestone of Cambro-Ordovician age, and located two 

 and a half miles northwest of Waynesboro, in Augusta county, 

 Virginia. He says : " A heavy dike of trap penetrates the 

 limestone on 'Mr. Steele's place, running in a N.W. and S.E. 

 direction, and passing a considerable distance in the Valley." 



In 1890, Mr. Dartonf described the occurrence of a small 

 group of basalt dikes in the Upper Silurian and Lower Devon- 

 ian beds in Highland county, lying west of Staunton, Virginia. 

 Mr. Barton's paper was accompanied by descriptive notes on 

 the petrography of the basalt dikes by Mr. Diller. 



In 1896, Mr. Dartonf extended his observations farther west, 

 which resulted in the discovery of additional basalt dikes, and 

 a very interesting series of acid dikes, classified by Mr. Keith 

 as " felsophyre." The acid and basic igneous material col- 

 lected by Mr. Darton was studied microscopically by Mr. Keith 

 and his results were incorporated in the former's paper. Two 

 of the dikes described were found in the adjoining portion of 

 Pendleton county, West Virginia. 



Keith recognized three distinct types of texture in the basic 

 rocks, which were the basaltic, the diabasic, and the porphy- 

 ritic. Considerable variations of texture characterized the acid 

 rocks. According to Mr. Darton, the dikes penetrate lime- 

 stones, shales, sandstones, and quartzites, which range in age 

 from Cambro-Ordovician to Lower Devonian, inclusive. 



While studying the manganese deposits along the western 

 base of the Blue Ridge during the summer of 1906, I was 

 attracted by the occurrence of a small isolated exposure of 

 basic igneous rock in the Valley Paleozoic sediments. A care- 

 ful examination showed the exposure of igneous rock to be in 

 the so-called Potsdam sandstone (Cambrian) and very near its 

 contact on the west with the Valley (Shenandoah) limestone. 

 This exposure is in Augusta county and about one mile north 

 of Basic, and about fifty paces from and on the east side of the 

 Shenandoah Valley division of the Norfolk and Western Rail- 

 way, near the crossing of the railroad by the wagon road. 



*The Virginias, iv, 45, 1883. 



fThis Journal, vol. xxxix, 269-271, 1890. {Ibid., vol. vi, 305-315, 1898. 



