98 UNIVERSITY OF COLORADO STUDIES 



the preservation of some of the strongest ridges. But since the peg- 

 matite dikes have been exposed to erosion far longer than the eruptive 

 dikes, the latter often stand out more prominently than the former. 



Sugarloaj Mountain is an excellent example of this. A strong dike 

 of latite passes through the mountain the slopes of which are deeply 

 buried beneath debris from the dike. At the top of the mountain the 

 dike reaches a width of at least 50 feet. However, since the sides of the 

 mountain are covered with slide-rock so deeply as to conceal the con- 

 tact with the country rock, the exact width has not been determined, 

 but an estimate of 100 feet would probably not be greatly in error. 

 Toward the west the dike can be traced but a short distance, and on 

 the northeast side of the peak it narrows to less than 20 feet within a 

 distance of one mile, notwithstanding that at this latter point erosion 

 has uncovered the dike 2,000 feet lower than its highest exposure. 

 The extreme width on Sugarloaf Mountain and the existence here of 

 smaller branches and shorter parallel dikes, mark this point as the 

 center of vulcanism for this particular dike. It is not improbable that 

 a fissure eruption once covered the surface with lava which protected 

 from denudation, for a long period, the older rock as well as the dike 

 itself. 1 



Although no lavaform rock can now be found in the region, it seems 

 hardly reasonable to attribute to the dike properties of resistance suffi- 

 cient to maintain its present elevation above the surrounding country. 

 Within three-fourths of a mile northeastward the same dike is less 

 resistant than the country rock, having weathered sufficiently to cause 

 a depression in the surface. Even the dike must have extended, 

 in comparatively recent times, several hundred feet higher than its 

 present limit, since great quantities of dike rock hang on the slopes 

 of the mountain and innumerable boulders have been carried long 

 distances out into the plains by Boulder Creek. 



1 In the hope that I might find evidence for this hypothesis I requested Dr. Cross to allow me to examine 

 the thin sections from pebbles found in the Denver formation, which he used in the preparation of his report 

 in the "Denver Basin Monograph" (Mon. 27, V. S. G. S.) This request was kindly granted, but yielded 

 only negative results. Among the slides examined there was apparently no effussive representative of the Sugar- 

 loaf rock. However, the pebbles from which these slides were made came from near Clear Creek rather than 

 from near Boulder Creek. Even had I found the rock for which I was searching the evidence would be very 

 remote. 



