342 B. WILLIS LEWIS AND LIVINGSTON RANGES, MONTANA 



mation is determined by the character of rocks and the superincumbent 

 load, the adjustment may consist in fracture, in flow and fracture, or in 

 flow, and at successive depths deformation related to the Lewis thrust 

 no doubt takes on the corresponding forms of crushing, folding, and 

 viscous flow. The meridional sector in which such shortening may take 

 place is indeterminate. It may correspond with the meridian of the 

 Lewis range or extend eastward or westward. General relations suggest 

 that a western sector moved relatively toward an eastern, which was sta- 

 tionary ; but the mechanical effects would be similar if an eastern had 

 moved against and under a western mass, or if they had both been in 

 motion each toward the other. The facts thus afford no certain basis of 

 conclusion as to the locus of deeper-seated shortening, corresponding to 

 the Lewis thrust. 



The relation of the Lewis thrust to the syncline of the Front ranges 

 presents interesting questions. Did they develop successively or simul- 

 taneously? If successively, the broad mechanical effects may have 

 been produced in either order, namely, the thrust first and the syncline 

 later, or vice versa. If simultaneously — that is, as effects of a single epi- 

 sode of compressive stress — the manner in which the force was distrib- 

 uted, partly in thrusting and partly in folding, may be definitely 

 analyzed. Let it. be assumed that as a result of the earlier compression 

 the Algonkian strata were bent in a synclinal flexure of less pronounced 

 character than the present one. Then, in the later compression, the 

 stress transmitted through the series was resolved into two components — 

 the one tangential, the other radial to the bedding. The former was 

 adequate to cause thrusting ; the other was competent to increase the 

 curvature of the beds. For a given stress the ratio of these two compo- 

 nents changed with the increase of curvature, the tangential component 

 losing as the radial gained. Thus thrust would cease when the tangential 

 became inadequate to overcome the resistance opposed to it, and curva- 

 ture would increase till the stress was taken up in the growing anticline 

 west of the syncline. One effect would be to concentrate folding along 

 the western margin of the syncline and to produce steeper dips toward 

 the northeast than those toward the southwest. Such is the fact, but it 

 might have arisen in other ways also. Another result would be to give 

 to the thrust surface a synclinal form, but one which would be less pro- 

 nounced than that of the overlying strata; and, again, the topographic 

 surface developed on Algonkian rocks during the Blackfoot cycle should 

 be depressed along the synclinal axis to an amount corresponding to 

 the degree of flexure suffered by the strata during the later compression. 

 This point is again referred to under physiographic problems. 



