QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 213 



morphosed and some greatly so. The investigations of Professors Smyth, 

 Kemp, Gushing, and Miller indicate extensive masses of syenite and 

 granite to be the most abundant rocks in the Adirondack region. The 

 granites are regarded as variants of the syenites. 26 



Miller has described the normal syenite of the Adirondacks as follows : 27 



"This rock shows a greenish-gray color when fresh and it weathers to a 

 light brown. Its weathered surface is seldom more than a few inches thick. 

 As regards structure and granularity, it is a quite variable rock. The granu- 

 larity ranges from fine to fairly coarse, with a medium grain decidedly preva- 

 lent. A porphyritic texture is sometimes moderately developed. The structure 

 ranges from only faintly gneissoid to very clearly gneissoid to almost schis- 

 tose, this structure being accentuated by the arrangement (or flattening) of 

 the dark-colored minerals, with their long axis parallel to the direction of the 

 foliation. Evidence of crushing or granulation is common, though it varies 

 greatly, the feldspars showing the effects of the granulation more than the 

 other minerals. 



"In mineral composition, too, the syenite is rather variable. Feldspars — 

 microperthite, orthoclase, and soda-rich plagioclase — constitute 50 to SO per 

 cent of the rock. Quartz, in varying amounts up to 20 per cent, is always 

 present. Pyroxene or hornblende, or both, occur in amounts up to 20 per cent. 

 The pyroxene is mostly a green augite, with sometimes a little hypersthene. 

 Hornblende is generally more abundant than pyroxene in the more quartzose 

 syenites. From 1 to 5 per cent of magnetite always appears. Small amounts 

 of zircon, zoisite, and apatite seldom fail. Garnet is much more sporadic in 

 occurrence, though at times it makes up several per cent of the rock." 



dishing 28 states that, when traced from place to place, the syenite is 

 quite a variable rock and becomes both more basic and more acidic than 

 the normal type. Both are chiefly peripheral changes, though they may 

 occur locally within the general mass. In its extreme basic facies the 

 normal quartz syenite is of gabbroic composition, while in its acid facies 

 variation into a rock of granitic composition with very abundant quartz 

 is indicated. Megascopic descriptions of the Adirondack syenites by 

 Gushing, Kemp, and Miller fit equally well the Blue Bidge type ; also the 

 variation in the amount of quartz in the normal type of the two regions, 

 which mark the passage into a rock of gabbroic composition on the one 

 hand and into one of granitic composition on the other, applies with 

 equal force in the Virginia region. 



20 For detailed descriptions of these rocks the reader is referred to the bulletins of the 

 New York State Museum ; also the following- papers should be consulted : 

 C. II. Smyth, Jr. : Bull. Geol. Soc. Am., vol. 6, 1895, pp. 271-274. 

 H. P. Gushing : Bull. Geol. Soc. Am., vol. 10, 1899, pp. 177-192. 

 H. P. Gushing : Bull. Geol. Soc. Am., vol. 18, 1907, pp. 477-492. 

 H. P. Cushing : Amer. Jour. Sci., vol. 39, 1915, pp. 288-294. 

 W. J. Miller : Jour. Geology, vol. 21, 1913, pp. 160-180. 

 W. J. Miller : Bull. Geol. Soc. Am., vol. 25, 1914, pp. 243-263. 



27 W. .1. Miller : Bull. Geol. Soc. Am., vol. 25, 1914, p. 245. 



28 H. P. Cushing : Bull. Geol. Soc. Am., vol. 18, 1907, p. 479. 



