214 KEPOET UNITED STATES GEOLOGICAL SUEVEY. 



to the overlying volcanics, the position of andesite, wherever it was 

 recognized as such, renaains constant. Frecjueutly later flows of hot, 

 lava have passed over the already rigid andesitic mass, and have baked 

 it very thoroughly. In this case the slabs of rock are often phonoli- 

 tic, and show innumerable changes of color, produced by a compara- 

 tively large admixture of ferric oxygen compounds. As a type of the 

 andesite of that region, we may regard that from Black Mountain. A 

 dark-gray, almost black, micro-crystalline paste contains minute crystals 

 of oligoclase. Rarely is the paste vesicular. Crystals of sanidite and 

 a black mica (probably biotite) are distributed throughout the mass. 

 Acicular crystals of hornblende are rare. Dpon weathering, the thin 

 slabs of rocks turn a reddish-brown, owing to farther oxidation of the 

 magnetite contained therein. 



In the eastern portion of main volcanic mass, which includes the Un- 

 compahgre group, a number of andesitic outcrops were noticed. As 

 might be expected, the phonolitic character of andesite is restricted to 

 mainly the upper horizon. It is owing, as can readily here be observed, 

 to a reheating by the subsequently-arriving trachytes. On the north 

 side of Saguache Creek this feature is especially noticeable. Long-con- 

 tinned, regular bluff's of trachyte overlie the andesites, and near the 

 iunctiou of the two the fragments of the latter produce a subraetallic 

 sound when struck. Here, too, as at many other localities, the ande- 

 sites are accompanied by a series of tuff's. 



All eruptions of andesite are purely massive. No truly volcanic erup- 

 tion has been observed in Colorado, but others, less massive than those 

 of andesite, of a peculiar type, occur frequently. In speaking of the 

 trachytes, more particular mention will be made of the types of erup- 

 tion and of the various ways in which they manifest themselves. 



TRACHYTE. 



It is the group of trachytes, above all others, that claims our atten- 

 tion. Not only are the varieties presented very numerous, but also the 

 modes of appearaut^e. Independent of the chronological successions, 

 we can appropriately divide volcanic eruptives in accordance with their 

 methods of protrusion. Such a division will not only furnish a clue as 

 to the general appearance of the volcanic rocks, but also — after their 

 character has once been recognized — to their geological age. 



The distinction between massive and volcanic eruptions has long since 

 been made, but it is inadequate to express all the most typical forms 

 that will be observed. Five types of volcanic eruptions may be dis- 

 tinguished : 



Massive^ denoting the fact that the lava was poured out through one 

 or more great fissures or openings, and upon reaching the surface im- 

 mediately spread in every available direction. 



Volcanic eruptions are such where the lava was propelled outward 

 through a cylindrical tube or narrow fissure. Incident upon this method 

 of eruption is the gradual formation of a crater and its accessories. 



Isolated eruptions may be characterized as such where the lava issued 

 from one or more openings, flowed slowly, and succeeded in building up 

 a mountain or group of mountains. No indication of craters or other 

 features of the volcanoes of the present day are noticeable. 



Awar/iac^ic* eruptions are those comprising one or more dikes or a 

 dike system. Narrow, sometimes long, fissures are formed in various 



*1 have coined this word in lieu of one expressing the main characteristics of the 

 group it designates. — E. 



