44 Transactions of the Royal Canadian Institute 



Western Basin. 



The northward continuation of this interior basin follows the western 

 edge of the Canadian shield. In Dakota and eastern Montana and 

 northward through the prairie provindes of Caniada it assumes a definite 

 basin form, in which is found above the Palaeozoic oil beds the Cretaceous 

 deposits which provide both coal and oil for the western states and very 

 abundant coal reserves for Canada. Owing to the great structural de- 

 pression in the central part of the continent to which the Palaeozoic beds 

 conform, the areas available for oil prospecting consist mainly of part 

 of a somewhat narrow rim along the eastern side of the basin that is in 

 a way limited to the part which projects above sea level. 



This assumption of a boundary to the production area is a tentative 

 one based on the fact that farther into the basin the depth of Cretaceous 

 sediments to be pierced becomes so great that drilling operations are 

 very expensive and very difficult. 



The re-appearance of the Palaeozoic rocks to the west of the basin is 

 generally in the form of fault blocks overthrust on the newer sediments, 

 and the actual western rim is deeply buried. There may be places where 

 these rocks could be reached outside of the fractured zone of the moun- 

 tains, but a great thickness of Carboniferous and early Mesozoic deposits 

 here overlie the Devonian, and experience in the American field points 

 to the greater value of the Cretaceous beds which overlie them, as oil 

 containers. Besides the surrounding rim, it may be of interest to note that 

 although there is little evidence of abrupt changes of slope or of struc- 

 tures favourable to the occurrence of probable oil reservoirs, one terrace 

 or broad anticline can be traced on the eastern slope of the basin from 

 the northern point southeastward about parallel to the mountains. It 

 is probably more sharply defined in the measures near the surface, than 

 in the Palaeozoic, and is discussed as affecting the Mesozoic deposits. It 

 is joined at almost right angles by an uprise, which extends north- 

 easterly from the Bow Island anticline, which rises to the south to join 

 the Sweetgrass arch, a structure running into Canada from the uplifted 

 and intruded area of northern central Montana. This ridge structure 

 which crosses the basin may be too deeply buried for present prospecting, 

 but its presence will be of interest should the Palaeozoic rocks of this 

 basin prove at any place to have oil pools. 



Northern Basins. 



No exact dividing line is drawn between the Western basin and those 

 basins to the north. The syncline of the Western basin flattens to the 

 north and the amount of depression in front of the mountains lessens. 



