Eggleston — Eruptive Rocks at Cutting 'sville, Vt. 403 



Aplite. — Narrow aplitic dikes of paisanitic quality 

 cut the coarse and medium phases of the augite syenite. 

 They also penetrate, for short distances, the country 

 gneisses to the west. 



Tinguaite. — The locations of tinguaite dikes are shown 

 in fig. 1. At the foot of the eastern slope of the south 

 knob of Granite Hill, along the bank of Mill River, a 

 complex of syenitic and essexitic phases is cut by numer- 

 ous narrow dikes of syenite-porphyry, tinguaite, and 

 camptonite. A tinguaite dike (D, fig. 1) towards the 

 upstream end of the exposure carries remarkably fresh 

 rock, selected for chemical analysis. The dike, about one 

 foot wide, dips 80° N. N. W. 



The hand specimen has a medium-gray color, a dense, 

 fine-grained texture, and a superficial resemblance to 

 blue quartzite, though of duller luster. A few, small 

 phenocrystic laths of feldspar are discernible. In the 

 groundmass, nephelite is sufficiently abundant to yield 

 much gelatinous silica when the powdered rock is treated 

 with hydrochloric acid. 



The thin section shows well-formed phenocrysts of 

 anorthoclase and microperthite in a fine, granular 

 groundmass. The groundmass carries some fairly dis- 

 tinct laths of feldspar and prisms of pyroxene. The 

 remaining feldspar, pyroxene, and most of the other min- 

 erals are allotriomorphic. Orthoclase and nephelite 

 appear in roughly equal amounts, with plagioclase 

 (albite) apparently somewhat less than either. Reckon- 

 ing groundmass and phenocrysts together, 90 per cent or 

 more of the rock is feldspar and nephelite. The nephelite 

 is seen to enclose minute orthoclase crystals. There are 

 occasional grains of cancrinite. A colorless mineral in 

 irregular and veinlike patches is probably sodalite; a 

 little chlorine reported in the analysis may thus be 

 accounted for. Pyroxene is represented by two varie- 

 ties. One is pleochroic, in colors from yellowish or 

 brownish to green. Extinction angles run up to 20°, 

 These characters, together Avith the proper elongation, 

 indicate an segirite-augite. The other, less abundant, is a 

 very pale or colorless variety, apparently diopside. The 

 accessories are magnetite, titanite, zircon and rare 

 apatite. 



The chemical analysis of this tinguaite (miaskose) and 

 its parallels are stated in the accompanying table : 



