LOW-ANGLE FAULTING 31 
ticular breaks near Rome, are especially numerous in the same 
relation to the overthrust throughout the southwest portion of the 
quadrangle. Somewhat analogous relations are to be noted else- 
where. Directly in front of the Lewis thrust in Montana there 
is represented on the structure sections a series of slice faults formed 
as if in preparation for another overthrust which presumably, if 
the deformation had been carried further, would have broken 
through lower than the Lewis slip and to the east of it.* 
Fic. 12.—Deformation of specially shaped mass. Slice faulting resulted. In 
this particular experiment the pressure blocks were not held rigidly in place by con- 
trolling flanges, but were free to rise or become tilted. 
A relationship between a piling up of rock masses and the 
development of the low-angle overthrust has therefore been 
suggested. It might at first seem possible that lateral thrusting 
applied upon the piled-up mass, thus bringing forces to bear in a 
higher plane than would be the case if there were no piling up, 
would, on the lever-arm principle, develop a rotational strain 
which would cause fracturing at a lowered angle. To test this 
question experimentally, there was molded in the box a homo- 
geneous mixture of clay and plaster, which was high adjoining 
both pressure blocks and low in the middle. The rectangular 
1 Eugene Stebinger, ‘‘Geology and Coal Resources of Northern Teton County, 
Montana,” Bull. 621, U.S. Geol. Surv., 1916, Pl. XV. 
