300 



DYNAMICAL GEOLOGY. 



the Yellowstone Park down the Snake River region. It spreads north over 

 Oregon and Washington. There are many other areas over the Great Basin, 

 to the south. No cones exist as centers of these floods of lavas, but to the 

 west are lofty volcanoes of the Cascade Range. In the Great Basin the 

 lavas of the areas are commonly rhyolyte. 



The two following figures are examples of surficial outflows and of parts 

 of Table Mountain, Cal., which resulted therefrom through denudation. The 

 melted rock flowed over the gravels and river-beds of the country, and thus 

 obliterated the old surface features. Subsequently erosion by waters, cutting 

 through the igneous layers, and then through the easily removable beds 

 beneath, left a flat-topped elevation. Such " table mountains," or mesas, are 



270. 



271. 



ic 



jfj^^ 



ilifiiifilM 



mwwmSSSfS miifiiimmM§lwmiistiiMi 



Sections of Table Mountain, Tuolumne County, Cal.: 270, at Maine Boys' tunnel; 271, at Buckeye tunnel. 



J. D. Whitney. 



common in California, Arizona, and some other parts of the Rocky Mountain 

 area. These fissures, as explained by Whitney, 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 ate 

 partly volcanic ash, and sometimes contain silicified stumps and logs ; and, 

 over all, the cap of basalt (&); bv 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. 



A stream of melted rock usually hardens more or less the bed of sedi- 

 mentary rock over which it flows ; or it bleaches, blackens, or otherwise 

 changes it. Should it change, in like manner, an overlying bed, this would 

 be evidence that the stream was not surficial but interstitial ; that is, an 

 intrusion between two layers. The hardening effects often fail, however, 

 because there was no moisture present ; for dry sands cannot be hard baked. 

 Moreover, coarse pebbly beds are consolidated more readily than shales, 

 because they let the steam, that may be generated from moisture, pass 

 through them, when the fine earthy beds do not. Hence the latter may 

 show little or no evidence of the heat. On these changes see further under 

 Metamorphism, page 312. 



4. Interstitial outflows. — The intrusion of the melted rock of a fissure 

 between the layers of the stratified formation it intersects may be either 

 a simple gravitational flow ; or a forced flow. 



(a) The melted rock will naturally flow from a fissure into any opened 



