STRUCTURE 341 



The Siyeh limestone, the Carboniferous limestone, or other stiff forma- 

 tion may elsewhere be found to have determined the thrust surface 

 within the old rocks. 



Associated structures of the Lewis thrust. — At the margin and beneath the 

 advancing overthrust mass, the effect should be to disturb a superficial 

 layer of greater or less depth. Strata might be overturned ; or, if rigid, 

 they might be pushed forward on a thrust plane parallel to the initial 

 thrust ; or in soft shales like the Benton, confused crushing might ensue. 

 The weight of the overriding Algonkian might be that of a few hun- 

 dred or of several thousand feet of rock. In Flattop the thickness is 

 1,800 feet or less ; in Chief it is 1,600 feet. Along the advancing margin 

 it would not in any case be that of the series of Algonkian strata ob- 

 served in the syncline of the Front ranges. The weight would not be 

 sufficient to restrain rocks from breaking, and the local effects of defor- 

 mation would be fracture, accompanied by irregular but extensive dis- 

 placement. These deductions in part correspond with the observed 

 conditions of the overthrust and underthrust masses. 



The surface on which the overthrust mass would ride forward would 

 practically coincide with the Blackfoot plain, with which the thrust sur- 

 face would thus become identified, but it is probable that in the process 

 the peneplain would be deformed. In underlying strata not taking part 

 in the displacement, the stress from which the overthrust resulted should 

 have been exerted to accentuate any previously existing flexures. The 

 Lewis anticline, if the structural antecedent of the Lewis thrust may be 

 so called, was probably paralleled by a gentler fold somewhat further 

 east, as a result of the earlier stage of compression. Any rise of that fold 

 during the later stage of compression would be expressed in arching of 

 the Blackfoot plain ; 'such arching as exists, in fact, if the peneplain and 

 the thrust surface coincide along the eastern base of Lewis range. An 

 experimental illustration of an overthrust, a parallel fold, and the coin- 

 cidence of thrust surface and subaerial plane may be seen in the sections 

 of model El, Mechanics of Appalachian Structure. 



The displacement observed on the Lewis thrust is 7 miles; the actual 

 displacement is probably greater. The superficial movement requires 

 accommodation of subterranean masses to an equivalent amount in 

 some manner. This adjustment does not affect the preceding discussion 

 of the development of the thrust, but it is a phase of the subject which 

 is naturally suggested in sequel. In section 4, plate 53, close folding of 

 strata is indicated in a position corresponding to a place beneath the 

 Lewis range, but both the manner of folding and the locus assigned it 

 is suggestive only. Applying the general rule that the nature of defor- 



