| OCR Text |
Show 76 GEOLOGY OF 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 exceedingly hard and much more efficient in effecting abrasion than granules 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 article. 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 valleys 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 w^ould 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 by itself seems to me sufficient to refute com- |
