12 The Ottawa Naturalist. [April 



port the animal life. The plains of North America bear in 

 their underlying rocks records of long invasions of the sea, and ' 

 these form a part of the history of a continent which seems to 

 have been a very old feature. 



Much of its early history is very obscure, but we know 

 that at several periods the ocean encroached and almost sub- 

 merged the continent. The maximum submergence was pro- 

 bably in Ordovician times, when much of the limestone de- 

 posits of the continent were formed. Later the seas seamed 

 to have been shallower, and the rocks formed by the debris 

 entering the sea were of a fragmental character, and became 

 better soil makers. The plains of eastern America owe most 

 of their fertility to the decay of these rocks, but the western 

 plains, now called the Great Plains, received still further treat- 

 ment beneath a shallow muddy sea which covered the sand- 

 stones and limestones of the former plain by a heavy coating 

 of mud now hardened to shale. Then when the sea invasion 

 was about over, the great mud flats supported a very rich 

 vegetation, which is preserved in coal seams. The later addi- 

 tions to the building of the plains consist of coarser material, 

 and indicate a nearer source of supply which means an eleva- 

 tion of the land underlying and adjoining the western edge of 

 the basin. With the draining away of the salt water there was 

 an additional elevation in the land area which amounted to 

 mountain building. This consisted of the formation of folds - 

 as a partial relief from the tangential strain, but as the move- 

 men continued, probably too rapidly for the material to follow 

 without fracture, most of the folds became broken. 



We thus find as a typical structure in the Rocky Mountains 

 fault blocks piled one against the other in regular succession, 

 repeating the same series of beds many times. In front of the 

 broken area, or to the east of it, folds and breaks of less in- 

 tensity and lower elevation occur at present, and towards the 

 east the decreasing disturbance in the rocks show very clearly 

 that the strain was from the west. The formation of the Rocky 

 Mountains is about coincident with the elevation of the plains, 

 for in their slow rise the soft rocks forming the covering of the 

 broken folds we 'e washed down and carried across the plains 

 by the streams or spread out in lakes. On the completion of 

 the first period of erosion, after the appearance of the outer 

 mountains, the plains p-esented probably a rather rough rock- 

 strewn surface on the higher slopes. The removal of much of 

 this debris was made possible only by a further elevation, and 

 with a steepening of the slope eastward the second scoring 

 began. This was continued until from the surface hundreds of 



