calkins. I Petrography of the John Day Basin. 11/ 



perhaps not altogether certain. The term granodiorite is pro- 

 visionally applied, although the proportion of qnartz appears to 

 be somewhat smaller than in the typical granodiorites of the 

 Sierra Nevada. The predominance of plagioclase over orthoclase 

 would seem to preclude the use of the term quartz-monzonite, 

 while the dark constituents are hardly abundant enough, to give 

 the rock a dioritic aspect. 



A small fragment obtained from a mine at Spanish Gulch 

 appeared to be similar in character to the rock just described, 

 but was too much decomposed to lie lit for microscopical study. 



Granite-porphyry of Spanish Gulch. — Although this rock 

 possibly has some genetic relationship with that just mentioned, 

 it is of decidely more acid character. The groundmass, which may 

 be seen with the pocket-lens to be a holocrostalline aggregate of 

 quartz and feldspar, carries numerous phenocrysts of quartz, 

 monoclinic, and triclinie feldspar, and a few tablets of biotite. 

 The yellowish brown color of the rock seems due mainly to the 

 dissemination of fine particles of limonite. 



By the aid of the microscope the phenocrysts are seen to com- 

 prise orthoclase, quartz, and plagioclase in about equal quantity. 

 The groundmass contains the same minerals, with quartz in 

 somewhat larger proportion than among the phenocrysts. The 

 biotite is of the common deep greenish brown variety. Apatite, 

 magnetite and a little zircon occur as accessories. The secondary 

 minerals consist of a little sericite and kaolin in the feldspars, 

 numerous specks of limonite throughout the rock, and specks 

 and veins of a greenish brown micaceous mineral that will be 

 described further in the sequel. 



The plagioclase phenocrysts exhibit, in addition to the albite 

 striation, frequent pericline and Carlsbad twinning, while one 

 Baveno twin was observed. A distinct zonal banding is always 

 visible between crossed nicols, and, although there is in general 

 an increase of acidity from centre to periphery there usually have 

 been oscillations during the growth of the crystal, expressed by 

 the fact that the successive shells are alternately more acid and 

 more basic . The maximum extinction angle observed in the zone 

 perpendicular to the braehypinaeoid was 12°, in the centre of a 

 crystal whose outermost and most acid zone extinguished at 2°. 



