302 WATSOK AND CLIXE IGNEOUS DIKES IX VIRGINIA' 



Page 



Chemical characters 330 



Mineralogical characters 333 



Textural characters 333 



Age relations of the igneous rocks 334 



IXTRODUCTIOX 



Although the Great Valley region, extending from Pennsylvania to 

 Alabama, inclusive, and lying immediately northwest of the eastern crys- 

 talline province, has been the subject of serious study by many geologists, 

 the essential though not entire absence of igneous rocks in general is 

 noteworthy. There are very few recorded occurrences of igneous rocks 

 found within the limits of this vast area! extent and thickness of Paleo- 

 zoic sediments. Naturally, therefore, the discovery of new occurrences 

 of igneous rocks within this region, although of common types, is of im- 

 portance: but when igneous rocks, belonging to types hitherto unknown 

 to the region and by no means common elsewhere, are found, their geo- 

 logic occurrence and petrologic character become of still greater scientific- 

 interest and importance. 



The occurrence of igneous rocks as dikes in the Paleozoic sediments of 

 middle western Virginia, west of the Blue Eidge, has been known for 

 many years. Fontaine, 2 John L. and H. D. Campbell, 3 Darton, 4 and 

 TTatson 3 have noted and described dikes of igneous rocks in this region. 

 Only two types, however, have hitherto been recognized, the most fre- 

 quent and common one of which has been diabase including basalt. In 

 addition to diabase, Darton has noted and described the occurrence of 

 granite-felsophyre in Alleghany County, Virginia, and several dikes of 

 basalt near the State line on the north, in Pendleton County, West Vir- 

 ginia. 



Eecent investigations by the writers, assisted by Prof. F. J. Wright, of 

 Bridgewater College, Virginia, have resulted in the discovery of numer- 

 ous other dikes of diabase in Augusta, Eockbridge, and Eockingham 

 counties, Virginia, and in addition to diabase four igneous rock types 

 hitherto unknown to the region, the four new types — quartz gabbro, 

 nepheline syenite, camptonite, and teschenite (feldspar monchiquite) — 

 occurring as a series of complementary dikes intimately and interestingly 

 associated with each other, cutting the Shenandoah group of limestones 



2 The Virginias, 1883. vol. iv. p. 45. 



3 McFarlane's Railway Guide. 1890. p. 361. 



4 Amer. Jour. Sci.. 1890. vol. xxxix. pp. 269-271 ; ibid.. 1898, vol. vi, pp. 305-315. 

 Monterey Folio. Xo. 61, Virginia-West Virginia. U. S. Geological Survey, 1899. 



5 Amer. Jour. Sci., 1907, vol. xxlii, pp. 89-90. 



