ASH FALLS FROM HISTORIC AND PREHISTORIC VOLCANOES 611 



no account is taken of the great thickness of ash that must have fallen 

 within 15 or 20 miles of the volcano. 



Ash falls from historic Volcanoes 



The intensity and violence of this eruption can be realized when the 

 amount of ash erupted is compared to eruptions in historic time. Sta- 

 tistics on volcanoes 6 show that the most violent volcanic eruption on 

 record was that of Tomboro, on the Island of Sumbawa, east of Java, in 

 1815, which, it is estimated by different investigators, ejected from 26.6 

 to 50 cubic miles of material. Krakatoa, in 1883, and Katmai, in June, 

 1912, each ejected about five cubic miles of material. During and after 

 the Katmai eruption its ash-fall was noted as far as 1,200 to 1,500 miles 

 distant. 



Ash-falls from prehistoric Volcanoes 



The occurrence in the southwest corner of South Dakota of clay beds 

 derived from volcanic dust is described by Wherry 7 as being widespread 

 throughout this section. During late Cretaceous and Tertiary times there 

 was intense volcanic action in the Rocky Mountain region, and these clay 

 beds formed of altered ash deposits are added evidence showing the in- 

 tensity of such volcanic activity at that time. 



A volcanic eruption comparable to any of historic time is described by 

 (Japps 8 as occurring in the upper Yukon. It is estimated that this ancient 

 volcano ejected about 10 cubic miles of material, all of which was de- 

 posited on land. 



Extent of Ash-fall during Lowville Time 



In an ejection of about 5 cubic miles of material, like Katmai, the ash- 

 fall was noted 1,500 miles away; so it is logical to conclude that in an 

 eruption in which ten times this amount of material was ejected, the ash- 

 fall would be noted at much farther distances from the volcano, and one 

 should expect that the entire Lowville Sea would show traces of volcanic 

 ash in its sediments, as its furthest point from this volcanic center was 

 about 900 miles distant, in the northwest corner of Wisconsin. It is use- 



6 G. C Martin : The recent eruption of Katmai Volcano, in Alaska. Nat. Geog. Mag., 

 vol. 24. no. 2, 1918. 



7 Edgar T. Wherry: Clay derived from volcanic dust in the Pierre in South Dakota. 

 Jour. Wash. Acad. Science, vol. 7, no. 19, 1917, pp. 576-583. 



* St^nhen I?. Oanps : An ancient volcanic eruption in the Upper Yukon Basin. U. S. 

 Geol. Survey, 1915, p. 95. 



