Related Geological Conditions 41 



every square kilometre, during the first thirty hours, and on 

 the 30th of June two hundred and nine kilogrammes in 

 twelve hours. During a much less important eruption of 

 the same mountain on the 3d of July, 1880, the amount of 

 volcanic dust ejected, according to Mr. Whymper, could 

 not have been less, and was probably vastly more, than two 

 millions of tons." 



But Russell's observations (op. cit. p. 285) specially 

 emphasize the great geological importance of such de- 

 posits. "In the Sierra Nevada, and over large portions of 

 the Great Basin, deposits of volcanic dust many feet in 

 thickness are frequently met with," and these, in part of 

 Pleistocene, in part of more recent age, become finer and 

 finer, with increasing distance from their source. Again 

 ( p. 288) he says: "In Alaska and adjacent portions of 

 Canada, still other extensive deposits of volcanic dust of 

 recent date are known. The writer, while journeying up 

 the Yukon River in 1889, observed above the mouth of 

 Pelly River a conspicuous white band from eight to twelve 

 inches thick, in the upper portions of the river terraces, 

 which was traced for fully two hundred miles. This de- 

 posit of remarkably pure volcanic dust had previously been 

 noted in the adjacent regions, and was more fully examined 

 by Hayes in 1881. The various observations show that 

 it occupies an area of fully fifty-two thousand two-hundred 

 and eighty square miles, and varies in thickness from a few 

 inches on its northeast border, to between seventy-five and 

 one hundred feet near its southwest margin. Its volume 

 has been computed by Hayes to be in the neighborhood of 

 one-hundred and sixty-five cubic miles. The volcano from 

 which this vast eruption of fine dust was derived, is as yet 

 unknown, but from its distribution, and its increase both in 

 thickness, and in coarseness toward the southwest, the point 

 of eruption is judged to be some seventy-five miles north- 

 west of Mt. St. Elias. 



This Alaskan deposit is pure white, except when im- 

 purities are present, and indistinguishable, at least In its 

 physical properties, from the similar material found so 

 abundantly in California, Oregon, and Washington." 



