SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 1 29 



exception of muscovite, and are also found in the calcite which replaces 

 the feldspar. Small apatites are present as inclusions in quartz, feld- 

 spar and mica phenocrysts. Primary magnetite probably occurs only 

 in the groundmass. 



RHYOLITE (FELSITE) 



Field relations indicate that this rock is later than the mica-dacite. 

 Nowhere within the area covered by the map do the dikes intersect, 

 but in Sunshine Canyon, about a mile from the western limits of Boulder, 

 dikes of the two rocks are in contact and parallel for several hundred 

 feet. Along the contact the rhyolite has been intruded into narrow 

 fissures in the dacite ; less often the later rock has caught up fragments 

 of the dacite, producing a breccia which is largely cement. 



The rhyolite dikes are generally narrow (10-30 ft.), but one dike near 

 Crisman has a width of 60 feet at one point. Exposures are often not 

 continuous. Moreover a dike may abruptly change its course, it may 

 fork, or it may be interrupted and continue its course, as an offset. 

 In color and texture the rock closely resembles the Leadville "white 

 porphyry," but it does not break into blocks under the hammer as does 

 the Leadville rock. White when fresh, it weathers often to a greenish 

 cast, and in places to pink. Concentric bands of pale brown at various 

 distances apart are not uncommon. The last product of decay is a 

 white clay which, as well as the less altered rhyolite, has been used to a 

 small extent in the manufacture of buff brick. 



Jointing is almost universal, but of different types. Prismatic 

 jointing with rectangular blocks is often seen, but this may give way 

 within a short distance, to jointing in which the only regular planes are 

 parallel to the walls of the dike, producing slabs and plates often a 

 small fraction of an inch in thickness. Dark brown or black dendrites, 

 oxide of manganese or a mixture of manganese and iron oxides, are often 

 present. 



Petrography. — The rhyolite carries a few megascopic crystals of 

 quartz, feldspar, mica and rarely pyrite in a white felsitic groundmass. 

 Not all of these minerals are likely to be found in a single specimen. 

 Quartz and mica are probably the commonest, while in many specimens 



