SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 99 



II. Rocks and Structural Relations 



The rocks of the district include gneisses and schists, massive and 

 gneissoid granites, pegmatite and aplite, quartz-monzonite, hypersthene- 

 diabase, trachyte, latite, hornblende-andesite, mica-dacite and rhyolite. 



GNEISSES AND SCHISTS 



Metamorphics occupy the western part of the area and extend many 

 miles beyond the limits of the map. For considerable distances the 

 contact with the granite may be well defined, but frequently weather- 

 ing has produced so much mantle rock that the contact is concealed; 

 in this case the boundary has been drawn where surface boulders indi- 

 cate its probability. Nevertheless, it is very unlikely that the line so 

 drawn is anywhere more than a few hundred feet out of place. Within 

 the area mapped as gneiss and schist, near the contact with the granite 

 there are numerous intrusions of the latter rock, sometimes several 

 hundred square feet in extent. Also near the contact, in the granite 

 area there are many large inclusions of the older rock. Where these 

 conditions are found the exposures have been mapped as metamorphic 

 or as granite according as the one or the other occurs in greater amount. 

 Probably nowhere is this transition zone more than a quarter of a mile 

 in width. 



The gneisses and schists, which have been developed through regional 

 metamorphism, are usually folded and often highly contorted. The 

 foliation is roughly parallel to the contact with the granite, although 

 locally, within a few hundred feet of this contact, the strike is approxi- 

 mately at right angles. The dip is everywhere high, rarely less than 

 6o° from the horizontal, and commonly away from the granite. 



Granitic Gneiss 



Granitic gneiss occurs over a far wider area in the district than any 

 other metamorphic rock. Quartz, feldspar and mica are the essential 

 megascopic constituents, with garnet and muscovite as occasional 

 accessories. Sometimes pyrite is seen in small grains, usually scattered 

 over the surface in the joint-planes. Almost every degree of foliation 

 can be found, from that of a coarse gneissoid granite to that of a finely 

 laminated mica schist. Lenticular masses of pegmatite, from a few 



