6 R. T. CHAMBERLIN AND W. Z. MILLER 
displacement. Though the fault plane locally has a dip as high 
as 50° owing to later warping, it averages about 15°." 
The Bannock overthrust, recently described by Richards and 
Mansfield, when traced through southeastern Idaho and Utah along 
its course, now made sinuous by erosion, has a length of approxi- 
mately 270 miles, and involves a horizontal displacement of not 
less than 12 miles. The thrust plane itself is a gently undulating 
surface nowhere steeply inclined, sometimes dipping to the east 
and sometimes to the west. If this slight plication be the result 
of subsequent folding, the shear plane must originally have been 
very nearly horizontal. 
In eastern Wyoming the Absaroka and Darby faults are really 
of the overthrust variety, although what remains of these planes 
shows a higher angle of inclination than most of the preceding. 
The fault plane of the Darby thrust is, in general, not far from 
parallel to the bedding of the overthrust sheet. East of Yellow- 
stone National Park the Hart Mountain overthrust is thought by 
Dake to show a displacement of not less than 22 miles, making no 
allowance for recession of the eastern front by erosion. Assuming 
average thickness for the beds involved, the vertical displacement 
is over 6,000 feet. 
In the Alps, so long and carefully studied, some of the most 
remarkable structures known to geologists are still in process of 
being worked out. As yet there is lack of perfect accord as to 
some of the features of their interpretation. They have commonly 
been interpreted as extraordinary and wonderfully drawn-out 
overfolds (nappes de recouvrement). Among certain geologists there 
has developed a disposition to substitute, in interpretation, over- 
thrust sheets of the Scottish Highland type® for these extreme 
1 Eliot Blackwelder, ‘“New Light on the Geology of the Wasatch Mountains, 
Utah,” Bull. Geol. Soc. Amer., XXI (1910), 517-42. 
2R. W. Richards and G. R. Mansfield, ‘‘The Bannock Overthrust, a Major Fault 
in Southeastern Idaho and Northeastern Utah,” Jour. Geol., XX (1912), 681-709. 
3 Alfred R. Schultz, ‘‘Geology and Geography of a Portion of Lincoln County, 
Wyoming,” Bull. 543, U.S. Geol. Surv., 1914, pp. 84-87, and structure sections. 
4C. L. Dake, ‘‘The Hart Mountain Overthrust and Associated Structures in 
Park County, Wyoming,” Jour. Geol., XX VI, No. 1 (1918), p. 50. 
5 Bailey Willis, ‘‘Report on an Investigation of the Geological Structure of the 
Alps,” Smithsonian Misc. Coll., LVI (1912), No. 31, pp. 1-13; also James Geikie, 
Mountains, Their Origin, Growth, and Decay, 1913, pp. 116-17. 
ps gy cmaieon atid 
som ail lla, amass mimmmaalcaalihl caiarear 
