VOLCANOES. 



735 



ity of Mount Hood, Oregon, cover many hundreds of square miles ; 

 and the thickness on the north side of it, where they are cut through 

 by the Columbia River, is, according to Le Conte, not less than 4,000 

 feet, the range in this part consisting almost solely of successive beds 

 of lavas. The name Cascade, applied to the Range, comes from the 

 cascades in the river along this cut. The rocks, as described, are 

 basaltic lavas above, and trachytes and porphyry below. 



Fig. 1118. 



Mount Shasta From a Photograph by Watkins. 



Besides these rocks, there are extensive beds of tufa, interstratified 

 with the lava stream, or making the surface deposits. The winds of 

 the region being mostly from the west the tufa deposits are largest to 

 the eastward of the Cascade summits. Mr. Condon speaks of travel- 

 ling over such an area, for 50 to 60 miles, and states that the volcanic 

 ash was evenly laid over the whole surface, like a covering of snow ; 

 and where attaining its greatest thickness, the sharp features of the 

 older surface ceased to show themselves through it. In many parts of 

 the Rocky Mountain regions, the tufas contain silicified stumps and 

 trunks of large trees (p. 709). On the north face of Amethyst Moun- 

 tain, in the Upper Yellowstone region, where the volcanic tertiary 

 beds are 5,000 feet thick, upright trunks occur at many levels, some 

 30 feet high, while prostrate trunks are met with 50 to 60 feet long 

 and 5 to 6 feet in diameter. (Holmes.) 



