OVERTHRUST FAULTS. 49 



of .'i horizontal force pushing \\ estward and a vertical force directed vertically 

 downward. The vertical force may be conceived to !><• the load of superin- 

 cumbenl strata; and if, as seems probable, there is a tendency of the plains 

 area to sink, under the load of accumulated sediments, with relation to the 

 adjoining unloaded mountain mass of Archean rocks, the relative effecl 

 won hi be the same as if the hitter had Ik -en subjected to a vertical upthrust. 



2. Ovcrthrust faults. — Tll6 Second Series 1(1' | ll l( •!!< )1 1 1( '1 1; I tO lie C< illsii l( Ti >< I are 



the longitudinal or strike faults in the Wyoming beds, \>\ which fragments 

 of the latter are left among the Archean rocks or a piece of the Archean 

 basemenl is pushed up among the red sandstones. The former pin mi om< -111111 

 is seen south of Smith Boulder Creels and :it several points along the 

 foothills to the north of the area mapped; the latter, a1 Smith Boulder 

 Peat and in the hogbacl? valley near Deer Creek. 



1 me of the latter limits is represented in section on fig. 7, p. 1 16, where 

 it is seen thai the fault plane dips to the easl and would, therefore, al first 

 glance, appear to be a normal fault in which the downthrow should be 

 t<> the east; but that it is in realitj an overthrusl limit maj be -ecu in 

 fig. "_'. ]i. 18, in which the pari of the section adjoining the fault is turned 

 so as to. bring the sedimentary beds to a horizontal position. In this dia 

 gram the line A I! represents the present surface as carved by erosion. 

 A F G II represents the ancient shore-line or contact of sedimentary 

 beds with underlying Archean, broken, however, bv the limit <i Gf', whose 

 movement has carried the point ('• to Gr'. 



In addition to the thrust movement, there has evidently been some 

 upward movement in the beds adjoining the Archean which brought aboul 

 their steeply upturned position, and this may be accounted for on the 



around that as this locality was so much nearer the shore-line than the 

 points where the sharp bend from the vertical to the horizontal position of 

 the bed occurs there would have been a lesser load of superinciunhent 



strata, either through nondeposition or l>v reason of subsequent removal by 

 erosion; hence the element of the compressive forci that tended to acl 

 vertically downward would have been smaller and the resultant force more 

 nearly horizontal, and thus have admitted of a certain amount of pushing 



up along the shelving shore. 



MOH XXVII 4 



