Dickerman and Wadsworth.] 28 [January 16, 



AN OLIVINE, BEARING DIABASE, FROM ST. GEORGE, MAINE. 



BY Q. E. DICKERMAN AND M. E. WADSWORTH. 



During the winter of 1879-80 two of the masters of one of the 

 Boston schools spent their spare time in studying microscopical 

 lithology at the Museum of Comparative Zoology under the direc- 

 tion of one of the writers of this article. One of these teachers 

 (Mr. Dickerman) became much interested in the above men- 

 tioned rock from the quarry of the Long Cove Granite Company, 

 situated near Tenn ant's Harbor, opposite Spruce Head, St. 

 George, Maine. He wrote out at the time a description of the 

 microscopic characters of the rock which has since been revised 

 by Mr. Wadsworth and is now published. 



The rock is sold in the markets of New York and Boston under 

 the name of " Black Granite." It is of a dark gray color, crystal- 

 line in structure, and shows numerous milk white striated feld- 

 spars. On the polished surface magnetite, pyrite, and feldspar 

 can readily be determined. In the thin section the rock is seen 

 to be composed of augite, olivine, feldspar, magnetite, horn- 

 blende, biotite, and pyrite. 



The feldspar is generally striated although some does not show 

 twinning when examined in polarized light. It forms the larger 

 portion of the section. While the feldspar is generally water- 

 clear, it is frequently somewhat kaolinized along the fissures and 

 in spots elsewhere. It contains in places numerous elongated, 

 staff-like, and lenticular black bodies, which were also seen in 

 minute rounded forms. These bodies are generally arranged par- 

 allel to the cleavage. The feldspar further contains inclusions of 

 augite, biotite, olivine, magnetite, and pyrite. 



The augite is in large irregular patches surrounded by the feld- 

 spar. The cleavage is in part the irregular Assuring of augite 

 and in part the parallel longitudinal cleavage of diallage, hence, 

 the rock in the common nomenclature might either be called dia- 

 base or a gabbro. The augite is quite full of the same black 

 bodies as the feldspar, which in places are so abundant as to ren- 

 der the crystal nearly opaque. The augite contains also magnetite, 

 pyrite, feldspar, and olivine. It is frequently altered to a coffee- 

 brown biotite and hornblende, especially on the borders, and 

 on the long tongues extending into the feldspar. 



