HO UNIVERSITY OF COLORADO STUDIES 



Glendale Granite 



This rock occurs in irregularly-shaped masses on both sides of 

 Lefthand Canyon in the vicinity of Glendale and Rowena. A good 

 exposure may be seen on the Lefthand Creek road less than one-half 

 mile west of Glendale, and another on the high road about three- 

 fourths of a mile east of Rowena. This rock is probably intrusive into 

 the older biotite-granite described above. Usually the exact contact is 

 not exposed, but underlies a talus-filled depression. This in itself is 

 evidence of an eruptive contact or plane of weakness along which weather- 

 ering has been more rapid than in either variety of granite. But there is 

 more direct evidence in the inclusion of small masses of the older granite 

 in the younger, and the gneissoid structure of the former near the con- 

 tact, while the Glendale granite is altogether massive. Furthermore, 

 the Glendale granite sometimes grades into pegmatite near the border, 

 the pegmatite ending abruptly with the gneissoid variety. 



The Glendale granite is usually a light-colored rock of which the most 

 striking feature is the great number and character of the phenocrysts. 

 These phenocrysts, which make up nearly one-half the rock, are ortho- 

 clase crystals tabular parallel to the clinopinacoid. They are commonly 

 less than 6 mm. thick and are from i . 5 cm. to 4 cm. long. Carlsbad 

 twins, readily seen on cleavage faces in reflected light, are almost uni- 

 versal. Quartz and small amounts of orthoclase and biotite with 

 occasional irregular grains of pyrite compose the groundmass. The 

 biotite is often slightly chloritized. 



Under the microscope the biotite does not resemble that of ordinary 

 granite in habit, but is in aggregates of very small flakes. It is doubt- 

 less secondary after some primary ferromagnesian mineral which under- 

 went alteration and recrystallization after the magma had come to rest. 

 What this primary constituent was, cannot be determined by an examina- 

 tion of the specimens collected; but it is possible that it was biotite as 

 suggested by the occurrence of primary biotite in small amount, as 

 inclusions in the quartz. The secondary biotite is greenish-brown 

 and strongly pleochroic. Muscovite and chlorite are common altera- 

 tion products with the separation of magnetite and hematite. Aside 

 from its crystal habit and persistent habit of twinning, the orthoclase 



