PETROLOGY 309 



GrENEKAL PETROLOGY OF THE IGNEOUS EOCKS 

 INTRODUCTORY STATEMENT 



In the region as previously defined there occurs a variety of igneous 

 rocks of both acid and basic character, the latter greatly predominating. 

 Their only mode of occurrence, so far as observed, is in the form of dikes 

 of variable size. Six types have been recognized, arranged in order of 

 decreasing silica, as follows: (a) Felsophyre, (b) quartz gabbro, (c) 

 nepheline syenite, (d) teschenite, (e) camptonite, and (/) diabase. 

 These are discussed below in the order mentioned. Their gneral distri- 

 bution and occurrence have already been discussed. 



When fragments of the nepheline syenite, teschenite, and camptonite 

 were powdered and boiled with dilute hydrochloric acid, filtered, and the 

 filtrate evaporated, a considerable amount of gelatinous silica was de- 

 posited. As developed in the microscopic description of these three rock 

 types, the minerals which yielded to the acid are nepheline in the syenite 

 and analcite in the teschenite and camptonite. 



ORANITE-FELSOPHYRE (TOSCANOSE) 11 



General statement. — Thus far this type of rock is restricted in occur- 

 rence to a series of small dikes in the vicinity of Monterey, Highland 

 County, Virginia. The dikes of felsophyre are more or less closely as- 

 sociated with those of basalt. Exposures of the acid rock are found to 

 the northeast, north, and northwest of Monterey, in the Alleghany Ridges 

 province, and have been briefly described by Darton, who was the first to 

 note them. They intersect or cut the Shenandoah limestone of Cambro- 

 Ordovician age, the Eockwood (Clinton) shales, and the Eomney shales. 

 The specimens collected by Darton were studied microscopically by Keith, 

 who classified the rock as "felsophyre." 



Megascopic character. — The rock of these dikes when fresh is light 

 gray or dove-colored and of fine-grained porphyritic texture. Pheno- 

 crysts of feldspar (albite and orthoclase), biotite, and augite, developed 

 usually with crystal outlines, are set in a fine-grained, light-gray ground- 

 mass. These minerals are subject to much variation in amount, plagio- 

 dasc and biotite being perhaps the most abundant, while augite usually 

 has scant development. As a rule the plagioclase (albite) phenocrysts 

 arc kaolinized and can thus be distinguished from the unaltered ortho- 

 clase in hand specimens. 



" Most of the facts relating to this type have been abstracted from the work of Par 

 ton and Keith: Amer. Jour. Scl., 1898, vol. vi, pp. 305 315; .Monterey Folio, No. til, 

 Virginia- West Virginia, U. S. Geological Survey, 1800. 



