718 



DYNAMICAL GEOLOGY. 



sents '• Gothic Mountain," in Colorado, in which a mountain mass of 

 trachyte rests on a base of Cretaceous rocks. In this nearly horizon- 

 tally stratified base, near the top, there is an independent dike of 

 the same rock, which was probably produced cotemporaneously with 

 the outflow making the mountain. The mountain is nearly 2,000 

 feet in height above the Cretaceous base, and 12,465 feet high above 

 the sea-level. The rock is trachyte, — a porphyritic variety, — and, 

 like that of many trachytic eruptions, is destitute, according to Hay- 

 den, of bedding or evidences of separate lava flows. 



Fig. 1117. 



Gothic Mountain, Colorado. 



A trachytic mass overlying Cretaceous rocks. 



These envptions through fractures are sometimes accompanied by 

 deposits of tufa, made of the lava that was shivered to powder by the 

 cold waters which the melted rock came in contact with. 



Large outflows of steam have frequently attended the outbursts, 

 which has penetrated the adjoining rocks, making portions of them 

 to look like scoria, and as described on page 419, often forming new 

 minerals in the walls of the dikes, or wherever the heat reached. 



3. Subordinate Igneous Phenomena. 



1. Solfataras. — Solfataras are areas where sulphur-vapors escape, 

 and sulphur-incrustations form. They occur away from intense vol- 

 canic action, where sulphur vapors and steam rise slowly. Incrusta- 

 tions of alum are common in such places, arising from the action of 

 sulphuric acid on the alumina and alkali of the lavas. A decom- 

 position of the lavas is another consequence ; it often results in pro- 

 ducing gypsum (or sulphate of lime), through the action of the 

 sulphuric acid on the lime of the feldspar or pyroxene ; also opal, 



