270 UNIVERSITY OF COLORADO STUDIES 



and is older than any other petrographic unit. Except where it is in contact with the 

 Evans Granite, its schistosity or banding has a prevaiHng dip to the north at every 

 possible angle. Its strike is east and west. Where in contact with Evans Granite, it dips 

 away from the granite with a strike parallel to the contact. In the intense metamorphism 

 resulting from pressure, it takes on the most fantastic and varied plications and crump- 

 lings possible, and every variety of folding to be found in any mountain region can, as a 

 rule, be found here within a restricted area. The folding is accompanied by faulting, but, 

 wherever possible to be determined, the resulting movements are for short distances only. 



The history of the rock would seem to be that of an igneous rock exposed to great 

 compression in the zone of flowage, and recrystalHzation of its materials along bands or 

 zones. A subsequent folding followed, and afterwards faulting and jointing. The 

 development of garnet and fibroUte described in the specimens is not universal, and the 

 fibrolite gneiss does not seem to be developed according to any particular law. The 

 garnets, on the contrary, have been observed in greatest development on the borders of 

 the granitoid gneiss, in the Evergreen Quadrangle on the east, south, and west of the 

 mass, though not on the north. This leads to the suggestion that the intrusion of Clear 

 Creek Gneiss has developed the garnets in the Fundamental Gneiss as contact minerals. 



The rock in the hand specimen is seen to be a fine-grained mixture of quartz, biotite, 

 feldspar, fibrolite, and garnet in a general banded arrangement. 



Microscopic. — Under the microscope the rock is seen to consist of a banded arrange- 

 ment of biotite, quartz, frequently showing pegmatitic intergrowths with the feldspar, and 

 undulatory extinction, and both microcline and plagioclase feldspar. Apatite is abundant. 

 In many specimens, especially those from the Evergreen Quadrangle, there is an abundant 

 development of fibrolite. This occurs as highly polarizing fibrous masses in parallel 

 bands with the other materials of the section, and even penetrating the biotite, following 

 the cleavage. Usually associated with the fibrolite is developed characteristic iron garnet, 

 often in great abundance. In addition, we frequently find magnetite, hematite, appar- 

 ently formed from the magnetite, rubellane mica, and occasionally hornblende, closely 

 interwoven with the biotite. The specimens have every indication of an eruptive rock, 

 greatly compressed and recrystallized, hence the banded structure. It nowhere shows any 

 signs of having been a sedimentary rock, and if it ever was, it has been entirely remelted 

 , to the complete destruction of all former texture. 



Clear Creek Gneiss 



This formation occupies a large portion of the area under consideration, and while 

 at some distance away from the Fundamental Gneiss it differs essentially from that unit, 

 there is rarely any sharp contact between the two formations. The two nearest approaches 

 to a sharp contact are to be seen in the eastern portion of the area studied. Here the 

 contact transition zone probably occupies a space of about five hundred feet. On North 

 Beaver Creek there is a space of about half a mile before either rock is prevailingly the 

 unit mapped. 



While the Clear Creek Gneiss shows the effects of having its banding or gneissic 

 texture developed by great compressive forces, it has nowhere, except perhaps in the transi- 

 tion zone noted on North Beaver Creek, been so much plicated as the Fundamental Gneiss, 

 neither has it suffered intrusion of pegmatite and diorite in so marked a degree as the 



