14 NEW YORK STATE MUSEUM 



portion of the quadrangle with fine exposures between Brant- 

 ingham lake and Partridgeville and also where the road leaves 

 the map east of Brantingham post office. The area between 

 Greig and Donnattsburg shows many large outcrops of roci<: 

 which here as well as around Partridgeville are more quartzose 

 than usual. Smaller patches of rather quartzose syenite are 

 shov\'n at Lyons Falls and in the river bottom east of Glenfield. 



The rock exposed in the quarry southeast of Denley may be 

 taken as typical of the best syenite of the quadrangle and a 

 detailed description of this type will now be given. In thin 

 section fresh feldspar is seen to be the most common mineral 

 which makes up 75 to 8ofo of the rock. Much of the feldspar 

 is present as microperthite usually in large crystals and with 

 the microperthitic structure beautifully exhibited. There is also 

 a considerable percentage of another feldspar, presumably anor- 

 thockse, which is characterized by a sort of moire or clouded 

 appearance. There is a small amount of plagioclase feldspar 

 ranging from oligoclase to andesin. The second most common 

 mineral is quartz of which there is 12 or 15;^. The quartz grains 

 are very variable in shape and size and are frequently broken 

 as a result of pressure. Of the dark colored minerals hornblende 

 and biotite make up about 5^ of the rock. The hornblende is 

 the common green variety Avith usual pleochroism and shows 

 frequent alterations to chlorite, i or 2^ of magnetite, some- 

 times with leucoxene borders, is also present. Beside these a 

 few small crystals of zircon, apatite, and zoisite may be seen nearly 

 always as inclusions. 



A study of the thin sections from the different syenite locali- 

 ties shows a range of minerals as follows : Feldspar 60 to Sofo 

 — microperthite always abundant, anorthoclase none to 20^, 

 oligoclase none to lofo] quartz 15 to 30^; hornblende none to 5^; 

 biotite none to 3^; magnetite none to 3^ and zircon, apatite and 

 zoisite are nearly always present in very small amounts. In 

 one or two cases a little garnet has been noted. The cataclastic 

 structure is always more or less well developed in the syenite, 

 sometimes being very prominent, which shows that the rock 

 must have been subjected to a pretty severe dynamic meta- 

 morphism. 



In Cushing's^ typical syenite at Loon lake and Smyth's^ typi- 



. ^ ■ 



^Geol. Soc. Am. Bui. 1899- 10:177-92. 

 * loc. cit. p. 473. 



