4:02 Eggleston — Eruptive Rocks at Cutting sville, Vt. 



those later dikes are exposed in the pit (P, in fig. 1) south 

 of the quarry (Q 2, fig. 1). They appear to cut essexite 

 and probably also biotitic pulaskite. 



Zircon-rich pulaskite porphyry. — At the foot of the 

 eastern slope of the north end of Granite Hill, about 200 

 feet downstream from a short tunnel (T, fig. 1), a dike of 

 coarse trachytic rock is exposed from the bank of Mill 

 River 40 feet up the steep slope. The dike cuts green, 

 quartz-bearing augite syenite, which is brecciated at the 

 lower contact and receives branch dikelets. The main 

 dike has a thickness of approximately 12 feet, and dips 

 about 50° S. E. About 125 feet upstream an irregular 

 dike of similar rock, with a maximum width of 15 feet, 

 extends 10 feet up the slope. Downstream from the main 

 dike, near a dam, similar rock is again exposed. All 

 three of these exposures may belong to the same dike. 



The rock of the main dike shows strong contact chill- 

 ing. The sections studied are of the coarse-grained, 

 middle phase. These specimens show broad, half-inch 

 tablets of gray feldspar, slender prisms and small grains 

 of black pyroxene, and less frequent, small, hexagonal 

 and irregular flakes of black mica, in a fine to medium- 

 grained mass of feldspar, black pyroxene, and black mica. 

 Feldspar greatly predominates, giving the rock its light 

 gray color. 



Thin sections show roughly idiomorphic feldspar 

 and pyroxene in a granular mass, including feldspars, 

 pyroxene, biotite, magnetite, zircon, titanite, apatite, 

 little hornblende, and possibly nephelite (or soda- 

 lite), corundum, and fluorite. About 90 per cent of the 

 rock is feldspar, chiefly anorthoclase, with orthoclase, 

 microperthite, and subordinate oligoclase (Ab G An t ). 

 Between and penetrating the feldspar are irregular areas 

 of zeolites or other secondary minerals, which may have 

 been derived from nephelite or sodalite. Augite, occa- 

 sionally with outer zones of asgirite-augite, or segirite, is 

 the principal colored mineral. 



Mineralogically the rock is a zircon-rich pulaskite 

 porphyry. Its complete chemical analysis was not 

 attempted. Dr. H. E. Merwin, however, tested it for zir- 

 conia in order to check the microscopic evidence of abund- 

 ant zircon. He found sufficient zirconia for at least 1 per 

 cent of zircon. He also determined the percentages of 

 soda and potash, as, respectively, 7-32 and 4-81. 



