SOME EXAMPLES OF ROCK VARIATION 375 



to others which incline to the ophitic texture, and to still 

 others which are distinctly micropegmatitic. The granitic tex- 

 ture is the most frequent. The color is uniformly light gray or 

 reddish, but at times, when the ferromagnesian minerals are 

 present in greater quantity, they become dark gray or greenish 

 brown. 



The important mineral constituents are feldspar, quartz, 

 hornblende, and biotite. Apatite, zircon, sphene, iron oxide, 

 and epidote (?) are the accessory minerals. The secondary 

 minerals present are white and brown mica, chlorite, epidote, 

 zoisite, calcite, and rutile. Plagioclase feldspar, orthoclase, and 

 microcline occur together. The plagioclase is present in indi- 

 viduals which are fairly automorphic. In the ophitic textured 

 diorites, the plagioclase is the best developed of any of the 

 essential constituents. In the granitic textured rocks the degree 

 of automorphism is highest where orthoclase and quartz are pres- 

 ent in the largest quantity, and diminish as these diminish in con- 

 sequence of the interference of the plagioclase individuals. For 

 the most part the plagioclases yield rather narrow sections, though 

 they can hardly be called lath-shaped. Measurements show the 

 plagioclase to be andesine. It is very much altered. Ortho- 

 clase is present in large plates, which form a part of the meso- 

 stasis for the plagioclase and the bisilicates. The quantity of 

 orthoclase varies considerably. It is very much decomposed. 

 Microcline is not abundant. It is very fresh, and in many cases 

 automorphic with respect to the orthoclase and quartz. Quartz 

 is present in variable quantity, and with the orthoclase forms 

 the mesostasis. The hornblende shows a good development in 

 the prismatic zone. The terminal planes are not so well devel- 

 oped. It varies from dirty green to reddish-brown in color. 

 The reddish-brown variety occurs in the center of the crystals 

 and is surrounded by the green. The differently colored zones 

 thus produced are not sharply delimited, and are also in perfect 

 optical continuity. No evidence is found indicating the green 

 kind to be an alteration product from the brown variety, and 

 they are both assumed to be primary. The biotite is brown, 



