494 NEW YORK STATE MUSEUM 



been highly metamorphosed and recrystalHzed. Thin sections of this 

 rock from Mount Tread way exhibit quartz, feldspar, hornblende, 

 biotite, piedmontite, magnetite and ilmenite. There is every evi- 

 dence of intense metamorphism. The quartz is strained, with undu- 

 latory extinction. The feldspar is mainly microcline, with sub- 

 ordinate plagioclase, and both feldspars show strain effects. Anor- 

 thoclase is often present. The plagioclase is usually oligoclase, but 

 both albite and labradorite are sometimes found. Large pink 

 garnets are sometimes present. The biotite is a very black variety, 

 pleochroic from black to pale brown, and often exhibiting pleochroic 

 halos. The piedmontite is small in amount, but is present in nearly 

 all the slides examined of this type of rock. 



Considerable variation in the amount of metamorphism is tO' be 

 seen in this series, and some variation in the relative quantities of 

 constituents. On Third Brother the maximum of strain is reached ; 

 some of the feldspars are bent through a large angle, and all feld- 

 spar and quartz show undulatory extinction. Microcline is the pre- 

 dominating feldspar. Piedmontite is abundant. Several brecciated 

 zones were found on the sides of the mountain and near its top; 

 at first sight they resembled serpentine dikes, but closer study showed 

 them to be small shear zones. These fault breccias showed pre- 

 vailing secondary minerals, quartz, epidote, chlorite and related 

 minerals. Faint traces of hornblende were made out, nearly altered 

 to chlorite with finely divided epidote and calcite. The plagioclase 

 was completely altered to kaolin and saussurite. Some magnetite is 

 present. Precisely similar rock occurs northwest of Penfield pond, 

 and on the hill northeast of Paradox lake. The commonest type 

 is somewhat less strained, with hornblende and biotite in about 

 equal amounts, and with microcline, plagioclase and orthoclase in 

 decreasing order of abundance. Quartz is always the most common 

 mineral. 



On the mountain erroneously called Trumbull on the map (the 

 local name is Ellis) is found the least altered variety of this rock. 

 No microcline was present, but a larger proportion of orthoclase 

 altering to kaolin and zeolites. The quartz was less strained; the 

 biotites showed no halos ; no piedmontite was found. 



