286 TRANSPOETATION OP EOCK DEBEIS 



keep bare tlie coarse materials of the western surface by blowing 

 away the sand and clay, while the protected bluffs on the east 

 collect upon their slopes all the jBlying particles from the prairies 

 above. 



The finely comminuted rock dust blown from volcanic vents 

 is often drifted for long distances by atmospheric currents, and 

 ultimately deposited in beds of no insignificant proportions. 

 Dense clouds of such dust were blown from Icelandic volcanoes 

 to the coast of Norway in 1875, and subsequent to the eruption 

 of Krakatoa (in 1883) the ship Beaconsfield of Philadelphia, 

 while at a distance of 831 miles from the source, sailed for three 

 days through clouds of dust which fell upon her decks at the 

 rate of an inch an hour. That such are not or have not in 

 the past been unusual instances is shown by results obtained 

 by the Challenger Expedition, volcanic ashes and sand being 

 repeatedly dredged up from almost abysmal depths at points 

 in the central Pacific far remote from land areas. The day 

 following the explosive eruption of St. Vincent, in 1812, the 

 Barbadoes Island, 80 miles to the windward, was completely 

 shrouded in darkness for many hours, the light of the sun being 

 almost wholly obscured by the cloud of impalpable dust which 

 in the form of a slow, silent rain fell over the whole island. 

 ^'The trade wind had fallen dead; the everlasting roar of the 

 surf was gone; and the only noise was the crushing of the 

 branches snapped by the weight of the clammy dust. About 

 one o'clock the veil began to lift, a lurid sunlight stared in 

 from the horizon, but all was black overhead. Gradually the 

 dust cloud drifted away; the island saw the sun once more, 

 and saw itself inches deep in black, and in this case fertiliz- 

 ing dust."^ The late eruptions of St. Vincent and Martinique 

 (1902), as described by numerous writers, furnish still more im- 

 pressive illustrations of the enormous amount of detrital material 

 ejected during a single period of eruption, and of its wide dis- 

 tribution. 



^Kingsley, as quoted by Belt, in The ISTaturalist in Nicaragua, p. 354. 



