ENDUCH.] ACIDIC VOLCANIC ERUPTIVES POEPH. TRACHYTE. 241 



It may be noticed tbat, in a general way, the various groups have a 

 combined strike parallel to tbe main strike of tbe great volcanic mass 

 of tbe Uncompabgre and fcSan Juan Mountains. Tbe trend of tbose 

 groups not far distant from tbe main volcanic area may be regarded as 

 being at angles approximating 90^ tbereto. In contradistinction, those 

 tbat are located far off have a trend nearly north and south. This ar- 

 rangeaienfc, though the connection may not appear perfectly clear, seems 

 constant, and must therefore have some bearing upon tbe correlations of 

 the two classes of eruptives. 



II.— PHYSICAL APPEARANCE. 



Dependent, in a measure, upon their location is the exterior appear- 

 Huce presented by these groups. They rise to considerable absolute 

 altitudes from tbe comparatively low country surrounding them. Mount 

 Wilson reaches the highest elevation among them, and is followed by 

 the Spanish Peaks. In conformity with their structure, tbe mountains 

 composed of porphyritic trachyte show unique exterior feature. Taking 

 a general view, they may be described as isolated groups rising abruptly 

 from the level of the surrounding country. Sharp summits character- 

 ize tbe peaks. Erosion has been productive of severe corrugation. 

 Eidges and spurs leading up to the highest points are sharply defined, 

 very often falling off precipitously on either side. Steep slopes are no- 

 ticeable, especially near the tops of the high peaks. A linear arrange- 

 ment of the most prominent points may be observed, due to the strike 

 of the points or fissures of ontHow. Within a certain distance from the 

 groups, and within their limits, the dikes have produced characteristic 

 forms. Walls, or isolated masses of rock, and steep, hogback-shaped 

 hills denote their presence. 



Usually the colors exhibited by the rocks are dark. Bright shades of 

 red and green are not wanting, however, owing to the presence of ferric 

 oxygen-compounds. 



The entire habitat of the porphyritic trachyte groups is so striking 

 that, if once seen, it will rarely be forgotten. All the special features 

 mentioned above combine to produce an effect tbat is unlike any other 

 observable in the same region. 



III.— STRUCTURE. 



Two types of structure may be distinguished : 



(1.) Mountains, or groups of mountains, are formed by tbe agency of 

 a definite arrangement of numerous dikes. 



(2.) Large masses of volcanic material have been ejected through one 

 or more apertures, have been accompanied by a stratigrapbical disturb- 

 ance, producing arched or dome-shaped flexures of sedimentary beds, 

 and have formed, aided by erosion, mountains and mountain-groups. 



To the first series belong such occurrences as have been observed at 

 West Spanish Peak, Park View Mountain, and Mount Eicbard Owen. 

 A radial arrangement of fissures was subsequently filled by hot volcanic 

 material. While breaking up, in part, tbe sedimentary beds through 

 which the lava passed, it has, at other points, enabled it more success- 

 fully to withstand erosive influences. 



Although it cannot be, and is not, claimed that solely erosion has pro- 

 duced the present relative elevation of such mountains, it has been a 

 very important factor in their genesis. Furcation, crossing or reticula- 

 tion of the dikes has resulted in tbe formation of points or series of 

 16 G 



