552 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



F. M. Endlich in 1875, and from his manuscript report for 1875 (in press) 

 I condense the following description. 



East Spanish Pealc is entirely composed of trachyte, rising high above 

 the surroundiog country. On the east and northeast, the Dakota sand- 

 stones are seen dipping to the eastward, and penetrated by dikes radi- 

 ating from the peak. On the wes and south, Carboniferous red sand 

 stones prevail, also penetrated by dikes radiating from the same center 



West Spanish Pealc is probably the result of a dike directly connected 

 with the mass of trachyte forming the East Peak. The trachyte has 

 evidently spread laterally on the summit, and rests on changed sand- 

 stones of Carboniferous age. Bach spur leading from the peak marks 

 the presence of a dike radiating from a common center. The longest of 

 these dikes have a general direction north and south. 



The present shape of the Spanish Peaks is due largely to erosion, the 

 igneous material resisting erosive influences more readily than the sur- 

 rounding softer sedimentaries. 



Six miles west of the West Peak, the Dakota group of the Cretaceous 

 and remnants of the overlying shales are seen standing in vertical posi- 

 tion. The sedimentary strata all around the peaks have not only been 

 much disturbed, but have also been subjected to considerable metaraor- 

 phism. All that can be said about their age is that it is Post-Lignitic 

 (Tertiary), as some of the dikes extending southward from the West 

 Peak penetrate Carboniferous, Cretaceous, and Lignitic strata. 



The rocks present considerable variety, but are generally porphyritic, 

 and consist of a feldspathic base, inclosing crystals of oligoclase, sani- 

 dite, mica, and acicular hornblende. 



HUERFANO AREAS. 



Northwest of the Spanish Peaks and southwest of the Huerfano River 

 are several small areas of the same porphyritic rock that is seen in the 

 Spanish Peaks. They seem to occur as masses that have come up 

 through fissures in the Dakota sandstone, and then spread out on the 

 eastward-dipping strata. Only one of the areas has radial dikes like 

 the Spanish Peaks, and this area, with another which lies immediately 

 west of it, are on the Carboniferous sandstones, the Dakota group touch- 

 ing the trachyte only on the north. 



At the southern extremity of the Greenhorn Monntains, on the north 

 side of the Huerfano River, is another small area of eruptive material 

 similar to those on the south. It rests partly on Triassic strata and 

 partly on granitic rocks. 



These areas will be fully described in Dr. Endlich's report for 1875. 



MIDDLE PARK AREAS. 



In Middle Park are two trachytic areas, which, from the character of 

 the rock and the mode of occurrence, are included with the eruptive 

 mountains, although they form comparatively low hills. The difference 



