Basalt in Paleozoic Rocks in Central Virginia. 307 



deeply decomposed. It is possible that Nos. 21, 22, and 23 are 

 along one line of fissure. If so, it is one coincident with the 

 crest of one of the highest anticlinals of the central Appalachian 

 region. No. 21 appears to be in the same fissure as the basalt 

 dike No. 9, but the continuity of the two rocks could not be 

 traced, owing to woods and debris. Further details of the 

 occurrence and geologic relations of these dikes will be shown 

 in the Monterey folio of the Geologic Atlas of the United 

 States now in course of publication by the Geological Survey. 



Notes on the Petrography . 

 By Arthur Keith. 



The eruptive rocks above described by Mr. Darton comprise 

 two classes of very distinct nature and appearance. The first, 

 or basic type, is similar in all respects to the basic rocks occur- 

 ring in dikes and sheets in the Jura-trias sediments and to a less 

 degree in the Paleozoic and Archean rocks. The first material 

 obtained in this region was described by Mr. Diller."* Subse- 

 quent investigations by Mr. Darton revealed additional dikes 

 and the existence of a second class of eruptives which are first 

 described in this paper. This later discovered group* is of a 

 decidedly acid composition and more nearly approaches the 

 granite-rhyolite than any of the other large rock families. 



Appended is a chemical analysis of a typical sample of the 

 acid rock, as determined by W. F. Hillebrand in the Laboratory 

 of the U. S. Geological Survey. For comparison a partial 

 analysis of a typical basic rock is adjoined as determined by the 

 same analyst. 









No. 18. 





No. 5. 



SiO a 







69*56 





43-38 



TiO„ 







•31 







A1A 







15'52 







Fe 2 3 







1-67 







FeO 







1-19 







MnO 







•07 







CaO 







1-20 





14-02 



SrO 







trace 







BaO 







•10 







MgO 







•41 







K 2 







4-68 





•56 



Na 2 







4-46 





1-64 



Li 







trace 







H 2 below 110° 



c. 



•34 







H 2 abo 



ve 110° 





•67 







Pfis 







•08 







co 2 







none 







s 







trace 







CI, Fl, 







not tested 



for 







101-26 





* 



This Journal, 



vol. xxxix, pp. 270-271. 





