PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 6 1 



grained, granitic rocks, showing feldspars, augite, and a few 

 quartz grains. The feldspars are apt to be epidotized. In thin 

 section they show a granitic structure, and are composed of a 

 rather basic plagioclase, about Ab x An 2 , nearly as much ortho- 

 clase, considerable quartz, stout prisms of pale green diopside 

 which is commonly uralitized, some pale green biotite also 

 altered, little or no magnetite, and rare small apatites. As all the 

 specimens were badly decomposed I made no analysis of them, 

 but they presumably approach in composition the nordmarkites. 



Porpliyritic diorite. — A peculiar rock which may be called a 

 diorite is found, along with the orbicular syenite, in similar 

 rounded masses enclosed in granite, near Bass Rock. It is 

 dark-brown, with a rather fine-grained granitic groundmass of 

 white feldspar tables and hornblendes, often surrounded by white 

 feldspar zones, lying in a finer-grained matrix. Through this 

 are scattered large phenocrysts of labradorite, up to 5 cm in 

 length, of a peculiar clove-brown color and thick tabular habit. 

 The cleavage of these is very good, and on basal cleavage sur- 

 faces fine twinning striations are visible. 



Under the microscope the groundmass shows a granitic 

 structure, composed largely of alkali-feldspar, with some plagio- 

 clase, small grains of colorless diopside, pale brownish-green 

 hornblende, rare biotite flakes, considerable magnetite in grains 

 and small stout rods, quite abundant apatite and no quartz. The 

 large phenocrysts show well developed twinning lamellae, which 

 give extinctions corresponding to a labradorite of the composi- 

 tion Ab 3 An 3 . They are dusty with minute liquid inclusions hold- 

 ing movable bubbles, and also carry inclusions of diopside, horn- 

 blende, and magnetite. An analysis was made of a rather coarse- 

 grained piece which was chiefly phenocryst, and this gave: 



Si0 2 - - - 54.99 CaO - 9.87 



Ti0 2 - - 0.29 BaO - - - none 



A1 2 0, - - 25.58 Na 2 - 4.95 



Fe 2 3 - - - 0.43 K 2 - ... 1>n 



FeO - - 2.70 H 2 - 0.38 



MnO - - - trace 



MgO 0.72 IOI -° 2 



