124 UNIVERSITY OF COLORADO STUDIES 



Specimens of this rock examined were too few in number to warrant a 

 complete description, and it is possible that more thorough examination 

 of material from exposures toward the west would result even in a 

 modification of the nomenclature. 



HO RNB LENDE-ANDESITE 



Three-fourths of a mile northeast of Glendale, just within the limits 

 of the map, hornblende- andesite occurs in a single narrow dike. Erosion 

 has exposed the dike but a few hundred feet in a small tributary of 

 Lefthand Creek; westward it passes under the coarse granite and peg- 

 matite which forms the hill. The rock is much jointed, and the surfaces 

 of the joint blocks are weathered to gray or red. The latter color is 

 pronounced only where the hornblende is more or less completely 

 removed. 



The fresh andesite shows feldspar and hornblende phenocrysts in a 

 grayish-black lithoidal groundmass. The feldspars are tabular and 

 from 2 mm. to 8 mm. long. They are commonly yellowish-brown as 

 the result of weathering, yet are often highly lustrous. Many of the 

 brownish phenocrysts have cleavage faces which show finely fractured 

 grains of colorless feldspar with vitreous luster. Rarely, with the aid 

 of a lens, distinct striations may be seen on cleavage faces. The horn- 

 blende crystals are prismatic in habit, and for the most part under 3 mm. 

 in length but occasionally are twice as long. 



Under the microscope the hornblende phenocrysts are seen to be far 

 more numerous than the feldspars. They are of the basaltic variety 

 and usually with well-developed crystal outline and good cleavage. 

 Many of the crystals were broken while the groundmass was still molten 

 but the broken parts are but slightly separated. Resorption phenomena 

 are entirely absent. Pleochroism is strong with a range from dark 

 greenish-brown to yellowish-brown with a greenish tint. The extinction 

 angle is very small and only rarely reaches its maximum of 9 . Ortho- 

 pinacoidal twins are not uncommon. A few small apatite crystals are 

 enclosed in the hornblende phenocrysts. Magnetite in irregular grains 

 is closely associated with the hornblende but probably only as an altera- 

 tion product. 



