48 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The retention of these four series names implies some slight tax on the 

 memory but that drawback is much more than offset by the ease of grasping 

 and systematizing the many petrographic and stratigraphic facts which must 

 be reviewed before the constitution of the great geosynclinal prism is under- 

 stood. In view of the general lack of fossils throughout the belt, the differ- 

 entiation and correlation of the beds must be based on lithological properties. 

 The following description of each series includes a statement of the facts on 

 which is founded the writer's belief in the integrity of the whole sedimentary 

 field, one huge sedimentary prism constituting the staple rocks in the eastern 

 third of the Cordillera at the Torty-ninth Parallel. The summary of the 

 individual facts, as they are clustered in describing the four series, will further 

 well illustrate the systematic variation in the geosynclinal prism as it is crossed 

 from east to west. 



In each type section the formations will be considered in their natural 

 order, beginning with the oldest. The description will, in each case, be made 

 concisely and will be shorn of many items of fact which do not appear of 

 importance in the larger stratigraphic problem. The Purcell Lava formation 

 will te treated in chapter IX. 



The description of the four type series will be found nearly to cover the 

 stratigraphy of the different ranges from the Lewis on the east to the Yahk on 

 the west. In the Galton and MacDonald ranges there are bodies of fossilifer- 

 ous Devonian and Mississippian limestone which are properly parts of the 

 prism, but, having generally been eroded away, now form only quite subordinate 

 masses within the Boundary belt. These will be described in connection with 

 the account of the Galton series. The only other bed-rock sedimentary forma- 

 tion occurring between the Great Plains at Waterton lake and the Purcell 

 Trench at Porthill is a thick but local deposit of Tertiary fresh-water clays 

 and sands flooring the Flathead valley. This occurrence will be noted in con- 

 nection with the stratigraphic description of the Clarke and Lewis ranges. 

 The stratigraphy of the Selkirk system is much more highly composite than 

 any of the eastern ranges; its description will, therefore, be detailed only so 

 far as the Summit series and the underlying terrane are concerned, and will 

 then be interrupted by a chapter giving the results of correlating study on this 

 gigantic stratified unit, the Rocky Mountain Geosynclinal.* 



As an aid to clearness it may be noted, in anticipation of a later 

 chapter, that the Rocky Mountain Geosynclinal includes all the sedimen- 

 tary formations from the base of the Belt (pre-Olenellus) terrane up to and 

 including the Mississippian formation, as these beds are developed in the 

 eastern half of the Cordillera. The Lewis, Galton, and Purcell series represent 

 only a part of the whole prism, in each case the youngest exposed bed being 



* Following Dana (Manual of Geology, 4th edition, p. 380) the writer distinguishes 

 the geosyncline, the large-scale down-warp of the earth's 6urface, from the load of 

 sediments which may accumulate on the down-warped area. In the present report the 

 load of sediments will be referred to as a "geosynclinal prism" or, more briefly, as a 

 "geosynclinal." 



