102 UNIVERSITY OF COLORADO STUDIES 



Hornblende-Gneiss 



In various parts of the metamorphic area there are dike-like masses 

 of a dark, sometimes black, heavy gneiss whose principal mineral is 

 hornblende. Small shiny flakes of biotite, white feldspar, and rarely 

 quartz can also be seen with the unaided eye. Occasionally narrow 

 veins of pegmatite are present. The rock is from medium- to coarse- 

 grained, extremely tough and breaks with difficulty under the hammer. 

 Gneissoid structure is strikingly noticeable in the field, but not always 

 so apparent in the hand specimen. 



Whatever may have been the original rock from which the schists 

 and granitic gneiss were developed, the hornblende-gneiss is, almost 

 without doubt, the result of metamorphism of basic igneous rocks. 

 Since they do not appear to have been subjected to the intense dynamo- 

 metamorphism, or dynamo-chemical metamorphism, which has pro- 

 duced the granitic gneiss, it is possible that basic dikes were intruded 

 into the granitic gneiss after the latter had suffered metamorphism 

 for a considerable period. These dikes would thus be in a position to 

 feel the effects of subsequent strain and shearing possibly not more 

 intense than that which developed the granite-gneiss described 

 below. The presence of pegmatite veins, while far from conclusive 

 evidence, is at least suggestive of the intrusion of the basic rocks subse- 

 quent to the most important period of metamorphism. 



Microscopically the rock shows a considerable range in the relative 

 proportions of the subordinate minerals. These are biotite, plagio- 

 clase, microcline, orthoclase and quartz, with pyrite, black iron ore, 

 titanite and zircon as accessories. The hornblende is green and 

 strongly pleochroic, except in a few cases where it has altered to a 

 colorless variety with high double refraction. Twins are quite common. 

 Terminal crystal boundaries are seldom present, but the characteristic 

 outline is often seen in cross-sections. Frequently the hornblende 

 encloses magnetite, quartz and feldspar in poikilitic manner. The 

 last two minerals sometimes penetrate, or apparently gouge into the 

 sides of the hornblende crystals. Most of the hornblendes lie with their 

 long axes in parallel planes. 



Biotite in small amount is universally present, and sometimes inter- 



