Xo. 14. — Contributions from the Petrograpliical Laboratory of the 

 Harvard University Museum. 



VI. 



A Basic Dike in the Connecticut Triassic. ^ By L. S. Griswold. 



Occurrence. 



Loose boulders of the rock to be described in this paper have been 

 known for some time to Mr. S. "Ward Loper, but the rock was first found 

 in place by Mr. J. C. Graham of Wesleyan University while at work for 

 the United States Geological Survey. It occurs as a dike, exposed on 

 the outlet of Beseck Lake, about a quarter of a mile west of the Air 

 Line Eailroad at Baileyville. The dike intersects the shale just under- 

 lying the "posterior" or uppermost trap flow of the Triassic series of 

 Connecticut. A small fault of about six inches was noted in the dike. 



Physical Characters. 



The rock is dark colored, almost black, and dense. The specific 

 gravity (average of three determinations) is 3.036. It attracts attention 

 by the gi'eat number and large size of the black phenocrysts contained, 

 also to some extent by the small spots of a white mineral. The rock is 

 very fresh immediately below the weathered surface. 



Examined with a magnifying glass the phenocrysts are found to be 

 of augite, hornblende, and an occasional large biotite. The maximum 

 diameter of the phenocrysts is perhaps f inch (19 mm.). The spots 

 of white mineral give a brisk effervescence with cold hydrochloric acid, 

 so they represent secondary fillings of cavities by calcite. The calcite 

 frequently occurs in the phenocrysts as a decomposition product. 



Microscopical Examixatiox. 



With the microscope two divisions of tlie components of the rock can 

 be made: the phenocrysts, comprising perhaps a third of the total mass, 



1 Published with the permission of the Director of the United States Geological 

 Survey. 



VOL. XVl — NO 14. 



