272 UNIVERSITY OF COLORADO STUDIES 



Binary Granite 



This granite occupies a small area just north of Hines' and a much larger area south 

 of the Troutdale granite. No traverse yet made has reached the southern boundary of 

 this mass. As this granite is easily weathered, it is almost impossible to find good out- 

 crops, and it can be traced only by its residuals and bv an occasional cutting. In this way 

 we find the topography of the area occupied by the Binary Granite subdued, and relatively 

 rolling with well-rounded slopes, especially in comparison with the Troutdale Granite 

 area. 



Petrography. — In the hand specimen this rock consists essentially of an intimate mix- 

 ture of quartz and red feldspar. 



Under the microscope the quartz is seen to have an undulatory extinction. The feld- 

 spar is, as a rule, microcUne, with some acid plagioclase and occasional orthoclase. The 

 accessory minerals are apatite and biotite occurring in small amount. 



Central City Granite 



This rock, so far as known, occurs only on each side and a little to the south of the 

 head of Spring Gulch just south of the Central City Railroad Station. It is very clearly a 

 mass of granite intruded into the surrounding gneiss, with which it forms a sharp contact to 

 the north on the road to Central City. With the exception of a small hill on its most south- 

 erly extension, this granite is easily disintegrated and forms characteristic well-rounded 

 outcrops. 



Petrography. — In the hand specimen this granite proves to be a hypidiomorphic 

 arrangement of quartz, feldspar, and biotite. There is a suggestion of a banded arrange- 

 ment, which is more evident in the larger masses. 



Under the microscope the quartz shows the results of dynamic action by its undulatory 

 extinction. The feldspar is found to be nearly all an acid plagioclase, with some few 

 grains of orthoclase. There is an abundance of biotite, also a little hornblende. Apatite, 

 titanite, magnetite, and zircon occur in the usual form of these minerals. 



The rock is classed with the alkali granites or granitites. 



Evans Granite 

 This granite occupies a small area on the western portion of the Evergreen Quad- 

 rangle. It strongly resembles the Central City Granite, and is very likely the same petro- 

 graphic unit. No petrographic determination was made of this rock. 



Troutdale Granite 

 As can be seen from the map, this petrographic unit occupies a territory on either 

 side of Bear Creek, from a point just east of the town of Evergreen to the western boundary 

 of the Evergreen Quadrangle. It shows every evidence of being a true eruptive granite, 

 and in many cases along Bear Creek, at best just west of Troutdale, are seen apophyses 

 of granite in the Fundamental Gneiss. This has been called the Troutdale Granite on 

 account of its picturesque development at the resort of that name. It is probably, judging 

 from the comparison of hand specimens, the same granite that occurs at Georgetown, 

 Colo., though, so far as known, there is no direct connection. While not found cutting 

 the Binary Granite, there is good reason for thinking it is younger, especially as it seems 

 to divide the Binary Granite into two masses. The topography of the area occupied by 



