306 WATSON AND CXINE IGNEOUS DIKES IN VIRGINIA 



courses largely on the soft rocks, shale and limestone, which form the 

 valleys, and having avoided the harder and more resistant rocks, such as 

 sandstone and quartzite, which are ridge-forming. 



Distribution and geologic Field Eelations of the Dikes 



The dikes of igneous rocks forming the subject of this paper are en- 

 tirely limited in occurrence to the counties of Eockbridge, Augusta, 

 Eoekingham, and Highland, which are located in the extreme west-cen- 

 tral division of Virginia, partly in the Shenandoah Valley and partly in 

 the Alleghany Eidges physiographic provinces. None of the dikes have 

 been observed to extend farther east than the northwest foot of the Blue 

 Eidge, where they have been noted intersecting the Potsdam group of 

 Cambrian age. The accompanying geologic map, figure 1, shows the dis- 

 tribution of the dikes. In certain areas of the Alleghany Eidges, espe- 

 cially in . the vicinity of Monterey, 8 Highland County, the dikes are 

 numerous. 



Of the six rock t} T pes recognized, diabase is the most abundant and has 

 the widest distribution, being observed at intervals over the entire region 

 where the igneous material occurs. The minimum distance from the 

 most southerly to the most northerly exposure of the diabase is about 60 

 miles, and for an east-west distance of about 50 miles, extending from 

 the foot of the Blue Eidge almost to the Virginia-West Virginia bound- 

 ary. Exposures of diabase are noted at numerous points between these 

 extremes. On the other hand, the dikes of granite-felsophyre are re- 

 stricted in distribution to a limited area in the vicinity of Monterey, 

 Highland County, while those of nepheline syenite, teschenite, and camp- 

 tonite, together with a single dike of quartz gabbro, occur in a closely 

 associated series confined within comparatively narrow limits in the 

 northern part of Augusta County. The distribution of the dikes is 

 shown on map, figure 1. 



In no case has the igneous rock been observed as a continuous exposure 

 at the surface for any great distance, but the exposures appear at irregu- 

 lar intervals in lines which either extend entirely across the Valley prov- 

 ince or in shorter parallel lines to these. Thus far the maximum distance 

 for which a single exposure has been traced does not exceed 1.5 miles, 

 and as a rule the exposures are very much shorter, frequently being meas- 

 ured by only a few yards. So far as could be determined, the dikes over 





s X. H. Darton : Amer. Jour. Sci., 1890, vol. xxxix. pp. 269-271 : ibid.. 1898, vol. vi, 

 pp. 305-315. 



Monterey Folio. No. 61, Virginia-West Virginia. TJ. S, Geological Survey, 1899, 



