SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. IO3 



grown with the hornblende. In importance, plagioclase is next to 

 hornblende, but varies "widely in^amount in different dikes. Much, if 

 not all, is labradorite. It is commonly twinned after both albite and 

 pericline laws. In some sections the labradorite encloses minute brown 

 crystals apparently prismatic in habit. Microcline and orthoclase 

 appear only occasionally. Quartz is present in fairly large grains in 

 some specimens; in others it is in minute patches probably as an alter- 

 ation product. In the larger masses it exhibits phenomena identical 

 with those noted in the quartz of the granitic gneiss. Black iron ore 

 is present in all the slides examined. Granular titanite is more rare, 

 and occurs only in sections that contain a small amount of iron ore. 



It is possible, if not probable, that the original rock was a gabbro or 

 diabase, and in some exposures, in so far as the mineral composition 

 and structure are concerned, the present rock might well be called a 

 hornblende-gabbro-gneiss. 



GRANITES 



Within the area under consideration is a small part of the granite 

 batholith which extends many miles north and south. On the west 

 it is in contact with the metamorphic rocks; on the east, about a mile 

 beyond the boundary of the map, it passes under the upturned sedi- 

 mentaries of Carboniferous and later periods. In this district the 

 granites fall into two classes: (i), hornblende-bearing biotite-granite 

 often porphyritic, which has undergone a considerable degree of 

 metamorphism and through which run numerous pegmatite and aplite 

 dikes; (2), a porphyritic variety of biotite-granite intruded into the 

 older granite and which, because of its occurrence near the village of 

 Glendale, is here called Glendale Granite. 



Biotite-Granite (Hornblende-bearing) 



No attempt has been made to map the even-grained and the por- 

 phyritic facies separately since they grade into each other and have 

 crystallized from the same magma with essentially the same mineral 

 composition. Although the granite is often massive, gneissoid structure 

 is present over wider areas and is often pronounced. This applies 

 equally to the even-grained and the porphyritic varieties. The color 

 ranges from a light gray to bluish and dark gray, depending on the 



