a 
No. 14. — Contributions from the Petrographical Laboratory of the 
Harvard University Museum. 
VE 
A Basic Dike in the Connecticut Triassic. By L. S. Griswop. 
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 Railroad 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. 
PuysicaL 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 great 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 ? 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 ExaMINAaTION. 
With the microscope two divisions of the components of the rock can 
be made: the phenocrysts, comprising perhaps a third of the total mass, 
! Published with the permission of the Director of the United States Geological 
Survey. 
VOL. Xv1 —No. 14. 
