50 CP DAKE 
of four or five miles. Throughout this distance the Madison rests 
on the Red Beds. In Sec. 22, T. 55 N., R. 104 W., two prominent 
hills of Chugwater are capped with isolated patches of Madison. 
In Sec. 16, T. 55 N., R. 104 W., the trace of the fault plane is lost, 
probably because it passes wholly into the Madison, where it is not 
easily detected. This view is supported by the fact that the Madison 
in this region appears to be excessively thick, as though repeated. 
Not far from the center of T. 54 N., R. 102 W., occurs an 
isolated peak known as Hart. Mountain. It consists of a cap of 
several hundred feet of Madison limestone, entirely surrounded by 
late Cretaceous and Tertiary sediments. It has been described‘ 
as due to a circular fault. Because this mass of Madison is entirely 
isolated in outcrop, it is not possible to demonstrate the continuity 
of the major thrust, just described, to this point. But the suppo- 
sition hardly admits of doubt that Hart Mountain constitutes a 
portion of the large fault block so widely exposed to the west, 
especially in view of the similarity of stratigraphic units involved. 
The extreme irregularity shown by the fault trace is largely 
due to erosion, in part to later deformation, since the fault plane 
dips at various but low angles at various points. 
The north and south extent of the fault has been proved for 
over 25 miles in a straight line and for more than double that 
distance measured along the sinuosities of its course. Exclusive 
of the Hart Mountain outlier the easternmost and westernmost 
exposures are separated by a distance of 7 miles; including Hart 
Mountain, by about 16 miles. At the westernmost exposure 
the fault passes beneath the Tertiary andesites and is lost. At 
this point the Madison rests on the Fort Union (?), which in turn . 
can be traced without break at least 6 miles farther west. If the 
movement was from the west eastward, as will be shown later, the 
fault plane must pass at least this far west, hidden below the lava, 
but cut through by erosion before the lava was poured out. If this 
is the case, the amount of displacement must have been not less 
than 22 miles, making no allowance for recession of the eastern 
front by erosion. Using average figures for the thickness of the 
beds involved, the vertical displacement is over 6,000 feet. 
tC, A. Fisher, Joc. cit. 
