proceedings: geological society 383 



A preliminary computation of the depth of compensation was made 

 in the Survey, using gravity observations alone. The results were not 

 entirely satisfactory, as the determination was weak. The speaker stated 

 that the geodetic evidence shows definitely that large areas, such as that 

 of the United States, are perfectly compensated on an average and that 

 areas very much smaller than the United States are also in a nearly 

 perfect state of isostasy. It will need further and extensive geodetic 

 observations to determine the minimum area which may be completely, 

 or in large part, in a state of isostasy. 



Stratigraphy of the Montana group ( Upper Cretaceous) in northwestern 

 Montana and its relation to the Belly River beds and Judith River formation: 

 Eugene Stebinger. The section of the Montana group, in north- 

 western Montana, as exposed on Two Medicine River between its mouth 

 and Family post office, was found to differ considerably from the stand- 

 ard section of the Montana group for central Montana, which, as de- 

 scribed by Stanton and Hatcher in 1903, in connection with their strati- 

 graphic study of the Judith River formation, includes the Eagle, Claggett 

 Judith River, and Bearpaw formations. The section on Two Medicine 

 River was readily divided into four lithologic units, which from the base 

 upwards are as follows: (1) Virgelle sandstone, a coarse gray to buff 

 sandstone 220 feet thick, resting on Colorado shale; (2) Two Medicine 

 formation, a rudely bedded mass of light colored clays and sandstones, 

 chiefly fresh water in origin, 1950 feet thick; (3) Bearpaw shale, a dark 

 marine clay shale, 500 feet thick ; (4) Horsethief sandstone, a coarse gray 

 to buff sandstone 360 feet thick. These four stratigraphic units were 

 traced northward into the area in southern Alberta containmg the 

 Belly River beds as described by G. M. Dawson, of the Canadian Geo- 

 logical Survey. The Belly River beds were fomid to be equivalent to 

 all of the strata included in the Virgelle sandstone and Two Medicine 

 formation. On the other hand, by tracing the formations in the section 

 on Two Medicine River eastward to central Montana, the Virgelle sand- 

 stone was found to be equivalent to the massive sandstone member 

 forming the lower part of the Eagle sandstone, while the Bearpaw shale 

 appeared to be the same at each locality. It thus became apparent that 

 the marine shale of the Claggett formation thins out in going westward 

 from the central part of the State, so that the Two Medicine formation 

 is equivalent to all of the strata in central Montana included in the 

 Judith River, Claggett, and the upper part of the Eagle formations. 

 The Belly River beds of Dawson are therefore not identical and equiva- 

 lent to the Judith River formation, as has been stated by Stanton and 

 Hatcher, but are equivalent to all of the Eagle, Claggett, and Judith 

 River formations combined. 



A diagram of the lithologic units of the Montana group in this region 

 was presented, outlinmg the sections described and also the section of the 

 Montana group from the Black Hills region, the rocks which are mainly 

 continental in origin being graphically indicated as distinct from those 

 that are chiefly marine. The interfingering of the sheets of the conti- 

 nental sediments from the west with the marine sediments from the east 



