Eggleston — Eruptive Rocks at Cuttingsville, Vt. 405 



The Southboro, Mass., tinguaite (also miaskose) contains 

 aegirite, while the Cuttingsville rock carries segirite- 

 augite. The Brazilian tinguaite is the classic example 

 from Serra de Tingua, the type locality. It differs from 

 the Cuttingsville rock in bearing leucite. 



Camptonite. — Camptonite dikes, not quite as numerous 

 as the tinguaite dikes (fig. 1), are associated most closely 

 with essexite and nordmarkite. 



Perhaps the most typical camptonite occurs in a dike 

 a foot thick, exposed in the wall of a small, abandoned 

 nordmarkite quarry (Q 5, fig. 1) on the lower north slope 

 of Granite Hill. The rock is black and porphyritic, with 

 a few scattered, flashing crystals of black hornblende, 

 and more numerous, smaller tablets and masses of gray- 

 white feldspar. One of the hornblendes is 2 centimeters 

 long. An occasional grain of olivine also contrasts with 

 the black, fine-grained groundmass. The thin section 

 shows phenocrysts of augite, moderately basic plagio- 

 clase, brown hornblende, and serpentinized olivine. The 

 groundmass is an aggregate of brown hornblende, plag- 

 ioclase, augite, magnetite, and some pyrite. 



In the more northerly of the two small eruptive areas 

 north of Granite Hill (fig. 1) three camptonite dikes cut 

 essexite or the country gneisses. One of the dikes, about 

 25 feet wide, carries many inclusions of gneiss, quartzite, 

 syenite, and probably essexite. They are generally 

 angular in shape and less than 1 cm. in diameter. The 

 camptonite is porphyritic. The phenocrysts are bril- 

 liant, black hornblendes. They are usually under 2 cm. 

 in length, though some reach 8 or 10 cm. A single large 

 grain of serpentinized olivine appears in one specimen. 

 The groundmass is dark gray, fine-grained, and dense. 



Thin sections show the phenocrysts to be brown 

 hornblende and, in smaller amount, 'zoned plagioclase, 

 andesine to basic labradorite. There is some kaoli- 

 nized orthoclase or microperthite, probably xenolithic. 

 Green pyroxene, more or less altered to actinolite and 

 chlorite, also appears among the phenocrysts. The 

 groundmass is chiefly composed of plagioclase laths, 

 small prisms of hornblende and actinolite,' and chlo- 

 rite. Brown biotite, magnetite, apatite, titanite, ilmen- 

 ite, and pyrite are accessory. 



