[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 103 



when explosive eruptions of a highly specialized alkaline magma took 

 place in a relatively localized area in the vicinity of the Crowsnest 

 pass, forming one of the very few manifestations of igneous activity 

 in the Rocky mountains of Canada (25). Immediately after this 

 volcanism a subsidence and a marine invasion took place, forming 

 shales which are called the Benton formation, but which may be sub- 

 divided on more detailed study. The quiet marine conditions of the 

 Benton were followed by shallow and in part freshwater conditions 

 during which the coarse,, crossbedded rocks of the Belly River forma- 

 tion were formed. Succeeding the Belly River stage a marine inva- 

 sion occurred during which the shales of the Bearpaw formation were 

 laid down, and this was followed by a stage in which conditions similar 

 to those of the Belly River recurred, during which the rocks of the 

 St. Mary River formation were accumulated, these being the latest 

 rocks of the Mesozoic era in this region (41,c) 



The rocks accumulated in the third consecutive era of sedimen- 

 tation, the Mesozoic, are thus seen to be altogether of shallow water, 

 and largely of freshwater origin, though three marine stages are 

 represented. In this era there was at least one widespread interval 

 of slight erosion, and there was also some local volcanism of relatively 

 small significance from a rock-forming point of view. The historical 

 interpretation of the stratigraphie facts as stated in this paper is at 

 variance with Willis' hypothesis (45, a) of a Mesozoic peneplain, and 

 corroborates Daly's view that "It would seem probable that during 

 the Mesozoic, this part of the Cordillera was never far above sea-level 

 (12, f). From the earliest Precambrian known in these Rocky moun- 

 tains to the Upper Cretaceous or early Tertiary, a stretch of time 

 representing most of the recorded geologic history of the globe, there 

 is no evidence in this region for more than relatively slight uplifts 

 and depressions, with no more deformation than that consequent on 

 differential subsidence. After the Upper Cretaceous or early Tertiary 

 (Fort Union) sedimentation had piled its load on the tremendous mass 

 of sensibly parallel strata underneath, a vast and relatively rapid 

 deformation affected the region, which is known as the Laramide 

 revolution. The effects of this revolution are now visible in the 

 severely folded and faulted sediments of the Rocky Mountain géosyn- 

 clinal, from the oldest known, the Waterton dolomite, to the youngest, 

 the Porcupine Hills formation. 



Different interpretations have been given with respect to the 

 date of the Laramide revolution in this region. Willis considers it 

 to have taken place as two episodes of compression, the earlier "not 

 earlier than Laramie time nor later than early Tertiary" (45, b) and 



