742 



DYNAMICAL GEOLOGY. 



are sections of Table Mountain, in Tuolumne County, California. 

 They show the old now buried river valley (cut out of tilted Sierra 

 schists, d), holding in the river bed (at a, a) auriferous gravel, and, 

 above, finer fluvial deposits (c), which often are partly volcanic ash, 

 and sometimes contain silicified stumps and logs ; and, over all, the 

 cap of basalt (b) ; b v is part of the outline of the adjoining modern 

 valley. Tunnels (t) are made through the " rim-rock " of such old 

 valleys to reach the gravel, the gold being collected in these bottom 

 deposits because of its weight. 



The following sketch, from Hayden's Report for 1873, represents 

 " Gothic Mountain," in Colorado, in which a mountain mass of tra- 

 chyte rests on a base of Cretaceous rocks. In this nearly horizontally 



Fig. 1121. 



Gothic Mountain, Colorado. A trachytic mass overlying Cretaceous rocks. 



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

 rock, which was probably produced contemporaneously 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 Hayden, of bedding or 

 evidences of separate lava flows. 



These eruptions through fractures are sometimes accompanied by 

 deposits of tufa, made of the lava that was reduced to fragments or 

 powder by the cold waters which the melted rock came in contact 

 with, or by cinder ejections ; and very often the beds of tufa consist of 

 a mixture of volcanic sand and ordinary sand or earth. 



Veins of minerals and ores are part of the results of these fissure 

 eruptions, as explained beyond. 



