wherry: clay from volcanic dust 581 



Origin. Any theory as to the origin of this clay deposit must 

 account for the peculiar features above noted, — the remarkable 

 continuity of beds for many miles along- the strike, the absence 

 of quartz, the presence of fresh feldspar and mica, and the por- 

 ous, colloidal character of the clay itself. It is believed that 

 these facts are adequately explained by the volcanic dust theory, 

 as here outlined. 



That there was considerable volcanic activity in the northern 

 Rocky Mountain region during late Cretaceous and Tertiary 

 times has long been recognized; and that volcanic dust ("ash") 

 was often carried far east over the Great Plains is well known. 

 The Tertiary dusts have been observed throughout Nebraska 

 and as far east as Plattsmouth, Iowa; 4 and in an informal com- 

 munication before the Geological Society of Washington, Dr. T. 

 W. Stanton has described an occurrence of volcanic dust in the 

 Cretaceous (Fox Hills) in southern North Dakota. 5 But erup- 

 tions yielding volcanic dust also occurred still earlier in the 

 Cretaceous, notably in Benton time, as recently emphasized by 

 Hewett. 6 On the occasion of the writer's visit to South Dakota 

 Prof. Freeman Ward, of the University of South Dakota, pointed 

 out a thin stratum of whitish material, which probably repre- 

 sents such an occurrence, in the Benton group as exposed in the 

 Missouri River bluffs in the northeast corner of Nebraska, oppo- 

 site Vermilion, South Dakota. There is thus no reason to ques- 

 tion the possibility that dust-producing eruptions occurred dur- 

 ing Pierre time and that this dust was transported as far as 

 what is now southwestern South Dakota. 



When volcanic dust falls on dry land it is likely to collect on 

 the lee side of elevations, the deposits being relatively thick in 

 proportion to their lateral extent and showing little or no strati- 

 fication. Numerous minute laminations may result, on the 

 other hand, from its deposition in shallow lakes. 7 But when 

 blown out over a body of deep water, such as the interior sea 

 tfyat occupied what is now the central part of the North Ameri- 



4 Barbour, E. H. Nebraska pumicite. Nebraska Geol. Survey, 4: 361. 1916. 



5 Journ. Wash. Acad. Sci., 7: 80. 1917. 



6 Ibid., 7: 196. 1917. 



7 Barbour. Loc. cit. 



