76 GEOLOGY OP THE HIGH PLATEAUS. 



rounded fragments of ordinary conglomerates have been ground and worn 

 away by the action of sand and grit carried in suspension by the water. 

 Now the ordinary arenaceous particles are quartz granules, which are 

 exceeding'ly hard and much more efficient in effecting abrasion than gran- 

 ules of softer material would be. But in a volcanic district, where the only 

 rocks yielding fine detritus are volcanic rocks, quartz sand is a scarce arti- 

 cle. The mud and fine stuff carried by the streams consist of fragments of 

 the rocks themselves, particles of feldspar, mica, hornblende, and still more 

 largely clay stained with iron oxide None of these materials possess the 

 hardness of quartz and their abrading power is consequently much less. 



The great magnitude of these formations is by itself a source of great 

 perplexity when we inquire as to their origin. Looking up from the val- 

 leys below to the vast palisades which stretch away into the distance, and 

 seeing that they are chiefly composed of this fragmental matter, we seem 

 to be face to face with an insoluble problem. How did all this material get 

 to its present position and whence came it ? That it was blown into the air 

 in a fragmentary condition and showered down into strata is an explanation 

 which becomes more and more untenable as our studies progress, and at 

 length comes to look quite absurd. These conglomerates are often seen 

 with a thickness of nearly 1,000 feet at distances ranging from 6 to 12 

 miles from the nearest eruptive focus, and filling all the intermediate space 

 between their outer boundary and the central eruptive mass to which we 

 look to find their origin. Prodigious as the projectile force of volcanoes is 

 known to be, there are no recorded observations which warrant the belief 

 that this force ever becomes so transcendent as would be necessary to hurl 

 such enormous quantities of fragments to such distances. The highest 

 velocity imparted to cannon-shot (over 2,000 feet per second) would be 

 trifling in comparison, and they would have to rise several times higher 

 into the atmosphere than the horizontal distances to which they would be 

 thrown. 



But supposing them to be showered down, let us try to imagine them 

 restored to the places from which the outrushing vapors or gases tore them. 

 What enormous vacuities we should be required to fill in order to replace 

 them all! This consideration In" itself seems to me sufficient to refute com- 



