SYNOPSIS 307 



limestone of the series and identified by Walcott as probably being 

 Beltina danai, the species of crustacean discovered in the Grayson shales 

 of the Belt mountains. The Algonkian series consists of limestone, 

 argillite, and quartzite, classified in five formations. There is a certain 

 degree of repetition in the general phases of sedimentation; limestone 

 is succeeded by argillaceous and quartzitic beds, which are surmounted 

 by a considerable thickness of highly ferruginous red sediments, and a 

 second great limestone also is followed by quartzite and argillite, the 

 last named being again of a deep red color and carrying casts of salt 

 crystals. There is apparent conformit}^ throughout. The series is so 

 situated with reference to other rocks that no lower or upper strati- 

 graphic limit could be determined. Dr G. M. Dawson classified the 

 strata as Cambrian, Carboniferous, and Triassic, but it is believed that 

 he mistook certain local overthrust faults for unconformities and was 

 misled by lithologic resemblances. 



Igneous rocks occur sparingly in the Algonkian series. An intrusive 

 sheet of diorite is extensive in the upper limestone formation and an 

 extrusive flow of diabase caps it. 



Carboniferous limestone, with an abundant fauna of the Saint Louis 

 horizon, was found west of the Front ranges. Cretaceous strata underlie 

 the Great Plains, and fossils of Dakota, Benton, and Laramie age were 

 collected from them. Early Tertiary conditions are represented by 

 erosion surfaces on the Great Plains, probably also in the Galton range, 

 and possibly in the Front ranges. Later Tertiary lake and marsh 

 deposits occur in the valley of North Fork of the Flathead. Preglacial 

 gravel beds were distinguished at a high level above existing drainage 

 channels. The drift is not discussed. - 



The Algonkian strata form a syncline whose axis trends'west.of north. 

 Southwestern dips vary from 5 to 30 degrees. Northeastern dips are 

 generally 30 to 40 degrees and locally approach or pass verticality. 

 Minor flexures within the syncline are very broad and low. The 

 northeastern limit of the fold is an eroded margin; the southwestern 

 is an anticlinal axis whose western limb is in part eroded, in part 

 thrown down by a normal fault along North Fork valley. Syncline 

 and anticlines are closely related to valley and ridge respectively, and 

 this relation extends, to heights of peaks. 



Along its eastern margin the oldest Algonkian formation rests upon 

 Cretaceous rocks. The outcrop of this abnormal contact is deeply sinu- 

 ous throughout the stretch from Saint Mary lake to Waterton lake. The 

 structure is described as an overthrust fault, on which the Algonkian 

 series has moved northeastward relatively over the Cretaceous rocks. 

 The displacement on the thrust surface is 7 miles or more, and the ver- 

 tical throw is estimated at 3,400 feet or more. The thrust surface dips 



