SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 107 



In places it appears to form a contact with the coarser granite on one or 

 both sides of the dike as well defined as that of any of the porphyry 

 dikes, and even contains what appear to be inclusions of the coarser 

 granite. But since the Salina dike is cut by pegmatite, and since in 

 other parts of the district the fine-grained variety can be traced into the 

 coarse-grained granite, it is inferred that it is a differential phase of the 

 same magma. 



Metamorphism— Over considerable areas gneissoid structure is so 

 marked and the results of dynamo-metamorphism so evident as to make 

 the term granite-gneiss the most suitable name for the rock. The fractur- 

 ing and undulatory extinction of quartz, the cloudy extinction of orthoclase 

 and the bending of biotite flakes are results of strain; a less conclusive 

 evidence is the fact that the potash feldspar has so generally assumed 

 the triclinic form. Just east of Summerville is an exposure of the por- 

 phyritic granite which has been sheared and squeezed so that the pheno- 

 crysts have been drawn out into lens-like forms and the groundmass has 

 become distinctly banded. In a few instances the gneissoid structure 

 may be a fluxional arrangement as indicated occasionally by the parallel- 

 ism of the feldspar phenocrysts in the porphyritic facies. But in 

 addition to the granite-gneiss mentioned there are many bands from a 

 few inches to scores of feet in width in which the parallelism of the 

 dark minerals approaches perfect schistosity, which cannot be explained 

 without assuming severe compression as an important cause. A finely 

 laminated chlorite schist occurs in a band about three feet wide, south 

 of the railroad a short distance west of Wall Street. The compression 

 that produced the schistose structure was probably the result of shearing 

 movement. Other evidences of movement are present, as in the many 

 faults, usually of small throw, best seen in connection with pegmatite 

 and aplite dikes. Slickensided granite indicates that movement of 

 considerable importance occurred along a north-south line passing 

 through Orodell on Fourmile Creek just east of the area mapped. The 

 bands of schistose rock mentioned above commonly trend north and south 



Metamorphosed granite in Sunbeam Gulch, on Logan Hill and west 

 of Sunshine, occurring in narrow streaks, differs from all the other 

 granite in the district in the high content of epidote and magnetite. It 



