BIOTITE TINGUAITE DYKE ROCK. CATA- 

 LOGUE NO. 960. 



BY JOHN H. SEARS, 



Curator of Geology and 3Iineralogy, Peabody Academy of Science, 

 Salem, 3Iass. 



OCCURRENCE, ETC. 



In the latter part of July, 1896, while investigating the 

 segirine syenite rocks at Manchester, Massachusetts, I 

 discovered at near low water mark on Gale's rocks, two 

 hundred yards south of Gale's point, a dyke of a very 

 peculiar color, and from a macroscopical examination I 

 decided that it was a new addition to the previously de- 

 scribed rocks of Essex County. The dyke is six inches 

 wide and is exposed for twenty feet. It is seen cutting 

 the augite syenite in a nearly horizontal position six feet 

 below the surface of the syenite mass. This outcrop is only 

 exposed at low water as at high water the entire sj'enite 

 ledge is submerged. The color of this dyke is, on the sur- 

 face, a grayish green, mottled with bluish-black spots, a 

 freshly broken surface is olive green color and the spots are 

 black. Its occurrence in the immediate region of the 

 segirine tinguaite dyke at Pickard's point,' "analcite tin- 

 gnaite," Dr. Henry S. Washington,^ and of the aegirine 



1 J. H. Sears, Bulletin Essex Institute, Vol. xxv, 4, 1893. 



» H. S. Washington, American Journal of Science, Vol. vi, pp. 182-187, 1898. 



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