4 PROCEEDINGS OP THE NATIONAL MUSEUM vol.71 



clinal axes and others of complex folding. But he could not work 

 out all details of structure in the time at his disposal. 



It is natural to inquire how nearly contemporaneous the periods 

 of faulting and folding, intrusion and metamorphism, may have 

 been. The zone of intense folding along an axis approximately 

 parallel to the basal contact zone crossing Mount Tilton extends 

 north and south from Italian Peak for 10 miles or more in each 

 direction, far beyond the limit of metamorphism and intrusion, and 

 it appears certain that the major structural feature is independent 

 of and earlier than the intrusive action while the metamorphism is 

 clearly associated with the latter. It seems inconceivable, however, 

 that the large intrusions can have occurred in such folded and frac- 

 tured rocks without adding materially to the structural complexity. 



It appears that the Elk Mountains fold is one element in the sys- 

 tem of orogenic movements which occurred at the close of the Cre- 

 taceous and continued into the Eocene. The intrusions of Italian 

 Mountain and the Sawtooth Range are considered as belonging with 

 the large number of stock, dike, and laccolith intrusions of similar 

 petrographic character occurring over a wide area in Colorado and 

 adjacent parts of Utah and Arizona. There is, however, no evidence 

 thus far noted to suggest that they are much younger than the 

 structural features. 



The great San Juan mountain area of volcanic and intrusive rocirs 

 lies south of the Gunnison Valley. Its lower volcanics extend down 

 to the Gunnison Canyon in some places and there meet similar if not 

 identical materials belonging to the West Elk Mountains breccia, 

 which forms an important mass extending as far north as the An- 

 thracite quadrangle. Thus it appears that the igneous history of 

 the Elk Mountains should be considered as but one local phase of the 

 great volcanic activity in southwestern Colorado, which in the San 

 Juan region continued at intervals through all Tertiary time. 



PETROGRAPHIC CHARACTER OF THE INTRUSIVES 



The petrographic character of the intrusive rocks in the area of 

 Figure 1 is simple and, in general, similar to that characterizing 

 many other occurrences in Colorado. The largest body has a nearly 

 imiform composition and texture wherever it has been examined in 

 the Crested Butte quadrangle for 10 miles west of Star Peak. It is 

 of varying shades of gray in color, evenly fine-grained, most mineral 

 particles ranging between 0.5 and 2 mm. in diameter. Plagioclase, 

 orthoclase, quartz, biotite, hornblende, with occasional green augite, 

 are the principal minerals. The lighter colored minerals predomi- 



