338 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



On the evidence presented in the preceding paragraphs it is assumed 

 that the Lewis thrust plane and the Blackfoot peneplain are one, at 

 least as far as the former is now traceable beneath the eastern spurs of 

 the Lewis range. 



To these assumptions may be added another based on broad observa- 

 tions of stratigraphy and structure, namely, that a structural effect of the 

 first magnitude in sedimentary rocks was originally conditioned by cir- 

 cumstances of deposition. This thesis was considered at some length in 

 Mechanics of Appalachian Structure* and in an article by Hayes and 

 the writer.f 



The three fundamental assumptions thus are: (a) The thrust surface 

 coincides essentially with the bedding of the Algonkian series, (b) It 

 coincides essentially with the highest peneplain on the Cretaceous rocks, 

 (c) The antecedent structures of the Lewis thrust were determined by 

 conditions of deposition. 



These assumptions being premised, the development of the several 

 stages may most readily be stated in proper order, from ancient to mod- 

 ern, and in simple assertive style. It may be understood that the writer 

 is conscious of the unproved character of some of the points. 



Antecedents by deposition. — The first stage considered is that of Creta- 

 ceous deposition (see section 1, plate 53). The Dakota epoch of the 

 Cretaceous period was one of wide invasion of the sea westward and 

 northwestward. The deposits in this region contain freshwater shells, 

 and may be considered as having formed in estuaries or lagoons. They 

 are succeeded by marine deposits (Benton), which may be followed by 

 other formations of the Cretaceous system, but of these only the Laramie 

 is identified. Dawson X gives a section of 8,290 feet of Cretaceous in the 

 foothills of the Rocky mountains. We have reason to think there are 

 more than 3,500 feet of Dakota and Benton under Chief mountain. The 

 surface beneath the Dakota formation was a plane; primarily- a pene- 

 plain; subsequently a surface of marine planation. It sloped gently 

 eastward, and was practically flat in its early history. As it subsided 

 and was buried under marine deposits, it was no longer flat, but curved 

 in gentle flexures, according to any inequalities of subsidence (see 

 section 2, plate 53). Variations in thickness of strata are among the 

 evidences of unequal subsidence, and had we good measures of the Cre- 

 taceous strata, we might demonstrate a point of first importance in the 

 hypothesis. As it is, we must rely on the special case of a shore. As 

 Cretaceons rocks do not occur west of the Lewis range in the region 



* Thirteenth Annual Report U. S. Geol. Survey, p. 253 et seq. 



t American Journal of Science, Third Series, vol. xlvi, pp. 257-268, Conditions of Appalachian 

 Faulting. 

 X Canada Geol. Survey, Report 1885, p. 166 B. 



