Darton and Diller — Basalt dikes in the Paleozoic. -<» ( .* 



Abt. XXXIII. — On the occurrence of Basalt Dikes in the 

 Upper PaUi riesin Central Appalachian Virginia: 



by II. Darton, l". s. Geological Survey. ' With 



notes <>n tin Petrography} by ,1. s. Djller, D. S. Geo- 

 logical Survey. 



During a recent detailed examination of the geology of the 



Appalachian region west of Staunton, Virginia, the writer 

 discovered a number of small basalt dikes traversing the Upper 



Silurian and Lower Devonian beds in Highland county, Vir- 

 ginia, and Pendleton county. West Virginia. 



The most conspicuous outcrops are about a mile SSW. of 

 Monterey, in a remarkable conical hill rising abruptly 60 to 70 

 feet above the valley of Lower Devonian black slates. Owing 

 to the deep disintegration and debris of the basalt, the outcrops 

 on the hill slopes are few, short and unsatisfactory, so that only 

 the more general relations could be determined. The dike 

 extends through the center of the hill and is a nearlv vertical, 

 irregularly lenticular mass varying from fifteen to twenty five 

 feet in thickness and bearing numerous small hranches. It 

 does not appear to have materially altered the enclosing shales, 

 or disturbed their gentle dip except in the immediate vicinity of 

 the dike. On the eastern side of the hill the contact is charac- 

 terized by the presence of large masses of breccia, consisting of 

 shale and sandstone fragments imbedded in basalt. The trend 

 of the dike is SW. and NE., while the strike of Appalachian 

 folds in this region is SSW. and NNE. At this locality the 

 basalt was not found to extend beyond the area of the hill, but 

 on the mountain slope, two miles westward, there is a dike 

 of similar material penetrating the Silurian (Oneida-Medina) 

 sandstone. 



The next occurrence observed was on the west slope of Jack 

 Mountain on the road to Doe Hill, Virginia, where an obscurely 

 exposed dike penetrates the Silurian (Lower Helderberg Niag- 

 ara) limestones along a NE. and SW. line and in trend with 

 the Monterey outcrops. 



Three miles east of this exposure is another dike in a knoll 

 two miles WSW. of Doe Hill, just north of the road to Jack 

 Mountain. This dike forms a prominent outcrop over a small 

 area, extending for a short distance across a Silurian-Devonian 

 (Oriskany) sandstone and Silurian limestone belt with a width 

 of about twenty feet. Its contact relations could not be 

 determined. 



Two other small outcrops wore found, both in the southern 

 part of Pendleton county, West Virginia. One is an obscure 

 exposure in Lower Devonian slates, crossing the road two and 



