THE HART MOUNTAIN OVERTHRUST 47 
and one-half inchin diameter. Red granite, basalt, quartzite, sand- 
stone, black chert, brown chert, and shale make up the bulk of the 
pebbles. The shale members of the formation are dominantly gray, 
but contain many red layers. Thin lignitic seams were noted, but 
no leaves were found in a sufficient state of preservation to permit 
identification. One thin seam of black coal was found in the 
formation. 
At one point the beds were seen to rest on the Cody with slight 
angular unconformity, although at several other points they par- 
take of the folding of the older formations. 
Though no fossils were found in these beds, the abundance of 
the red clays, the pebbly character, and the slight angular uncon- 
formity at the base all seem to favor correlation with what Hewett 
has called the Fort Union. It is true that Hewett has made no 
mention of the large concretions which are so abundant, and the 
writer noted similar ones from the basal Laramie of Fisher (Hewett’s 
Gebo), not far to the east of the area mapped. At the same time 
similar concretions were noted, however, at a much higher horizon 
in Fisher’s Laramie, in what is believed to represent Hewett’s 
Fort Union. Fisher describes such concretions as occurring in the 
Laramie, but does not indicate the exact horizon. 
Along the North Fork of Shoshone River these beds trace 
continuously into what Hague’ has mapped as Pierre and Fox | 
Hills. At the same point, however, Hewett? calls them ‘‘ Tertiary 
sandstones and shales probably of Wasatch age.” Structural 
reasons will be given later for believing that they are earlier than 
Wasatch. 
STRUCTURE 
The major thrust.—The main plane of fracture occurs at or near 
the base of the Madison (Mississippian) limestone, which has been 
thrust out over beds varying in age from Madison to Fort Union( ?). 
At the southernmost point, where the fault was located, the lime- 
stones rest on sandstones of Fort Union(?) age, but toward the 
north the stratigraphic throw decreases until at the northernmost 
™ Hague, Absaroka Folio, U.S. Geol. Survey, Folio 52. 
2 Hewett, ‘Sulphur Deposits in Park County, Wyoming,” U.S. Geol. Survey, 
Bulletin 450, p. 478. 
