576 wherry: clay from volcanic dust 



unless the radiation is gray, but is the so-called " color tempera- 

 ture." (5) In the case of intercomparison of black or gray 

 bodies perfect intersections of the log isochromatics occur for all 

 wave lengths, and the temperature at which the intersection 

 occurs is the true temperature of one of the radiators in ques- 

 tion. (6) It is theoretically possible to have two radiators at 

 different temperatures, one gray and the other far from gray, 

 with each showing identically the same spectral distribution of 

 energy. 



GEOLOGY. Clay derived from volcanic dust in the Pierre in 

 South Dakota. Edgar T. Wherry, Bureau of Chemistry. 1 



At several places near the southwestern corner of the state of 

 South Dakota there is found, interbedded in the Pierre forma- 

 tion, a peculiar clay. During the early part of May, 1917, the 

 writer made a field examination of the occurrence of this mate- 

 rial, and as a result of his observations, combined with micro- 

 scopic and chemical examination of the samples collected, has 

 come to the conclusion that this clay represents an altered vol- 

 canic dust, accumulated in the sea during Pierre time. 



Occurrence. The clay occurs at a number of places along a 

 line, convex toward the south and over 50 miles long, extending 

 from Buffalo Gap to Provo, around the southern end of the 

 Black Hills. It forms numerous beds varying from a fraction of 

 an inch to 4 feet in thickness, interbedded with and sharply 

 delimited from the black shale characteristic of the Pierre; the 

 latter here contains fish bones and the shells of various lamelli- 

 branchs and cephalopods, such as Inoceramus and Baculites, 

 clearly fixing its horizon. 2 The majority of the clay beds can be 

 traced practically continuously and without marked variation 

 in thickness throughout the whole of this distance; a very few 

 of them are lenslike and more limited in extent. Near Pedro, 



1 This paper was prepared while the writer was connected with the National 

 Museum. 



2 Specimens of these have been kindly examined and identified as Pierre types 

 by Messrs. J. W. Gidley, of the National Museum, and T. W. Stanton, of the 

 Geological Survey. 



