PETROGRAPHY OF THE ROCKS 361 



original minerals are entirely altered to secondary minerals in many of 

 the sections, but, with few exceptions, some trace of the original outline 

 of the feldspar constituent is shown and more or less of the original rock 

 texture preserved. While this is true for the feldspar constituent, the 

 original bisilicate constituent is completely altered in every slide studied, 

 without any indication as to what the mineral originally was. Con- 

 sidering the age and composition of the rocks, it seems remarkable that 

 any of the original minerals or structures should be preserved at present. 

 When shown, the texture varies from a partial microophitic to micro- 

 litic in the non-porphyritic types, with the same variation and composi- 

 tion of the grouUdmass in the porphyritic rocks denoted. 



The constituents present are plagioclase, bluish to light green am- 

 phibole, chlorite, epidote, zoisite, calcite, iron oxide (partly magnetite), 

 quartz, and apatite. Of these only the feldspar, a part of the iron oxide 

 (magnetite), and apatite are original. Both chlorite and epidote, inti- 

 mate^ associated with more or less hornblende, are abundantly devel- 

 oped in most of the sections, sometimes one, sometimes the other pre- 

 dominating ; but the two are at all times intimately connected. 



In the porphyritic and non-porphyritic types the feldspar is present 

 as lath-shaped crystals, showing the broad twinning lamella? of the 

 albite type. Twinning after the Carlsbad law was observed in several 

 instances. In the groundmass of the porphyritic rocks and in the fine- 

 textured non-porphyritic types the feldspars are microlitic, with the 

 boundaries less sharp and well defined than for the lath-shaped feld- 

 spars, and the twinning is not at all or only slightly indicated. Some- 

 times the feldspar grouping is suggestive of the sheaf-like arrangement 

 described by Clements * in similar volcanic rocks of the Hemlock forma- 

 tion of lake Superior. Poikilitic texture is well developed in many of 

 the larger feldspar laths. 



Feldspar is the only porphyritically developed mineral, and it con- 

 sists of fairly large, stout laths of broadly striated plagioclase, with maxi- 

 mum extinction angles measured on the twinning planes of 14 to 20 

 degrees, which would apparently indicate an acid plagioclase, probably 

 near oligoclase. 



The feldspars of both the groundmass and phenocrysts are frequently 

 fractured and mashed, showing the effects of pressure, which is seen to 

 best advantage in the schistose rocks. 



Not a trace of original hornblende was positively identified in anjr of 

 the sections. Amphibole is fairly abundant in most of the slides as a 

 secondary product, and as such is usually light green to slight bluish 



* Monograph no. xxxvi, U. S. Geol. Survey, 1899, p. 99. 

 LTII— Bull. Geol. Soc. Am., Vol. 13, 1901 



