BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 2, pp. 339-348 March is, 189i 



COMPOSITION OF CERTAIN MESOZOIC IGNEOUS ROCKS OF 



VIRGINIA. 



BY H. D. CAMPBELL AND W. G. BROWN. 



[Read hefore the Society December 30, 1890.) 



CONTENTS. 



Page. 



Generally uniform Composition of Mesozoic Trap 339 



Traps of exceptional Composition 340 



Varieties 340 



Localities of Occurrence 341 



Description of the Hypersthene-Diabase 342 



Constituents of the Hypersthene-Diabase 342 



Characters of the Hypersthene 344 



Associated Minerals 345 



Description of the Olivine-Hypersthene-Diabase 346 



Discussion 348 



Generally uniform Composition of Mesozoic Trap. 



No one has ever studied the earlier Mesozoic formation along the Atlantic 

 border without being struck by the numerous ridges and dikes of trap which 

 are found in connection with it, and no one has ever studied the trap found 

 in these ridges and dikes without being impressed by its uniformity of com- 

 position. 



J. D. Dana* has pointed out the wonderful uniformity of these eruptive 

 rocks, calling them all dolerites ; rocks made up essentially of labradorite 

 and pyroxene, with more or less magnetic iron ore in disseminated grains 

 or crystals. He says that the aspect of specimens from Nova Scotia to 

 North Carolina is closely the same, and that the density varies little from 3. 

 The lowest density which he quotes is 2.94, and the highest 3.16. 



Chemical analyses of these rocks by G. H. Cook,t W. G. Mixter,3; S. T. 

 Tyson,§ F. A. Genth || and G. W. Hawes^f all show their chemical composition 



* Am. Jour. Sci., 3d ser., vol. VI, 1873, p. 105. 



t Geology of New Jersey, 18(58, p. 215 ; Am. Jour. Sci., ad ser., vol. VI, 1873, p. 106. 



i Am. Jour. Sci., loc. cit. 



\Loc. cit., p. 107. 



II (ieoi. Surv. Pa., Report C, 1874, p. 122 ; and Report CCC, 1877, p. 275. 



\\ Am. Jour, Sci.. 3d ser., vol. IX, 1875 p. 185. 



L— Bull. Geol. Soc. Am., Vol. 2, 1890. (339) 



