306 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



Page 



Livingston anticline 331 



Lewis overthrust 33L 



Character and extent 331 



Warped thrust surface 332 



Structure beneath the thrust surface 333 



Structure above the thrust surface 333 



Structural antecedents of Lewis thrust ... 336 



Explanation 336 



Assumptions 336 



Antecedents by deposition 338 



Antecedents by folding and erosion 339 



Conditions of overthrusting , 340 



Associated structures of the Lewis thrust 341 



Date of the Lewis thrust 343 



North Fork normal fault 343 



Topographic relations 343 



Geologic relations , 343 



Date of normal faulting ... 344 



Structure and physiography 344 



Great plains and Front ranges.. 344 



Heights and anticlines 346 



Valleys and synclines 347 



Distinctive character of Front ranges 347 



Eecognition of peneplain in the Front ranges 348 



Igneous rocks of. the Algonkian series ; by George I. Finlay 349 



General comment 349 



Diorite .- 349 



Diabase 350 



Other igneous rocks 351 



Diabase and andesite boulders . 35 L 



Tinguaite boulders 351 



Synopsis 



The facts stated in the following article relate to the Front ranges of 

 the Rocky mountains in Montana and adjacent Alberta, between the 

 Great plains and the valley of North Fork of Flathead river. The 

 Front ranges are two — Lewis range, which, rising from the Plains across 

 northern Montana, extends into southern Alberta and ends, and Liv- 

 ingston range, which, lying 8 to 15 miles west of the Lewis, becomes in 

 Alberta the easternmost height of the Rockies. The features of adja- 

 cent districts are described so far as they bear on the main subject, the 

 stratigraphy and structure of the Front ranges. 



Lewis and Livingston ranges consist of stratified rocks of Algonkian 

 age, as determined on fossils which were found by Weller in the lowest 



