exdlich.] VOLCANICS. 207 



talline paste contains innumerable small crystals of epidote, replacing 

 hornblende. At some places the epidote congregates so as to form 

 narrow veins through the rock. Octahedra of magnetite are also found 

 in the paste, which has a dull gray color. Sanidite occurs only on very 

 minute crystals. Mica was not found at all. 



Of interest the ridge running south of Bear Creek from station 30 to 

 Sultan Mountain will be found. Although no sedimentaries appear in 

 Baker's Park, immediately across the ridge they set in, covered by tra- 

 chytes. Their elevation is quite considerable, reaching more than 10,000 

 feet at those points. Duringsorne former time they were disturbed, 

 and now the trachytes rest unconformably upon them. Sultan Mount- 

 ain, station 26, is the last massive peak of volcanic material on the 

 downward course of the Animas, and reaches an elevation of 13,366 

 feet. To the south and southwest the trachytic material shows regular 

 stratification, very nearly horizontal, showing only a slight dip south- 

 ward. At several points the process of erosion has separated small 

 patches from the main body of volcanic material. This is the case at 

 Engineer Mountain (station 31, 12,971 feet high). A little more than 

 one thousand feet of light-gray trachyte caps the dark-colored Carbon- 

 iferous sandstone. The shape of the mountain is that of an elongated 

 pyramid, with two very steep sides, the one of them nearly vertical. 

 On the latter side the trachyte shows a decidedly columnar struc- 

 ture, which contrasts sharply with the stratification of the sandstones 

 below. Lithologically the rock belongs to No. 4. It is a light-gray 

 crystalline paste, containing crystals of oligoclase dispersed through the 

 material. Upon weathering, the feldspar decomposes and falls out, giv- 

 ing it a vesicular appearance. Small particles of mica are distributed 

 sparingly. West of Engineer Mountain a sharp ridge extends from 

 north to south, again showing the capping of trachyte upon sedimentary 

 beds. With this the consideration of the continuous area of volcanic 

 material may be regarded as completed, but there are seve'ral isolated 

 eruptions occurring in the district ■surveyed. One of the largest and 

 most prominent is the Mount Wilson group (station 35), of which the 

 highest peak reaches an elevation of 14,280 feet. Immediately north- 

 east of this mountain is the probable point of outflow, spreading from 

 there to the east more particularly. Two of the highest spurs running 

 from the main mass are formed by the flows from Mount Wilson, and 

 rest upon sedimentary strata. Lithologically the trachyte of these 

 spurs would be referred to No. 3. East of the main peak is a curious 

 monument of trachyte, an obelisk-like mass of stone placed upon a nat- 

 ural pedestal, of symmetrical form. The formation of this monument, 

 which was named "Lizard's Head," is owing chiefly to the tendency to 

 columnar structure shown by the rocks. Its height amouuts to 290 feet, 

 while at the base its diameter is scarcely over 60 feet, Its summit is 

 13,160 feet above sea-level. Toward the outer edges of the group the 

 trachytic material is stratified, while on the highest point, Mount Wil- 

 son, no definite stratification can be observed. On the summit a rock 

 occurs, that may be referred to the Doleritic group. The rock is dark- 

 gray to black, containing a feldspar that very closely resembles labra- 

 dorite. Small nodules occur in it that seem to be composed of minute 

 hornblende crystals, varying somewhat in color. 



Another isolated group is the one upon which station 36 is located. 

 On the south side of the Bio Dolores a quantity of volcanic material is 

 f und, forming in shape a figure similar to that of the horseshoe. Sta- 

 tion 36 is one of the high points of that flow or series of flows, having 

 an elevation of 12,554 feet. From that, and probably two other points 



