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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Apr. 10, 



mineral composition, corresponding with that of the underlying 

 strata, without admixture of materials carried from a distance further 

 than a sprinkling of erratic blocks, of small size, and only to be 

 found crowded on favourable spots. 



These consist almost entirely entirely of fragments of metamorphic 

 rocks, limestone being very rare. 



I have not remarked the ordinary erratics at a greater altitude than 

 3000 feet; but at 3700 feet above the sea and 50 miles from the 

 Rocky Mountains, there occurs a very extraordinary group of blocks 

 of granite, resting on a high plateau formed of sandstone strata. 

 These blocks are of great size, one having been estimated to weigh 

 250 tons. Although lying in a line miles apart, they seem to consist 

 of the same rock — viz. a mixture of quartz and red felspar, the latter 

 predominating, with only faint traces of mica disseminated in very 

 minute flakes. They present smooth surfaces, although in general 

 they are rhomboidal in form. Some of them are cracked into several 

 pieces, which are quite detached, but are evidently parts of the same 

 block*. 



If these blocks were derived from the granitic belt to the east, as I 

 believe all the erratics of the prairies have been, they must have 

 travelled at least 400 miles. From the fact, however, that they are 

 beyond the western verge of the Drift, and that the boulders were 

 found, as a rule, to diminish in size in that direction, it maybe that the 

 presence of these blocks is due to veiy different agencies, — different, 

 at least, in the time of their occurrence. No granite was observed 

 on the east flank of the Eocky Mountains within British territory, 

 but the Trois Butes, south of the 49th parallel, are said to be com- 

 posed of granite, and also the Black Hills; but both of those lo- 

 calities are much to the south of where those blocks occur. 



The surfaces of the higher plains are in some localities traversed 

 by profound rents, resembling the valleys of great rivers, but which, 

 after running for several miles, are generally found to be closed at 

 both ends. They are often occupied by deep lakes of salt water, de- 

 pressed 200 feet to 300 feet below the plain, and from 500 yards to 

 a mile in width. The great coulees in the neighbourhood of the 

 Ear Hills, south of Battle Eiver, are the best examples of these ; 

 but they are found in man)- other localities. It is difficult to con- 

 ceive that they are due to erosion alone f. 



Before leaving the superficial deposits of the prairie-country, it is 

 necessaiy to notice the great river-valleys which traverse it, and 



* For mention of a similar phenomenon, see Hooker's 'Himalayan Journals,' 

 vol. i. p. 201. 



t The ravines mentioned by Sir Charles Lyell in his Second Journey in the 

 United States, p. 25. vol. ii., as occurring in the cretaceous and tertiary strata of 

 Georgia, seem to be very similar to them. He says, that when the woods are 

 cleared from the country, the sun, acting on the unprotected surface of the argil- 

 laceous strata, produces cracks that are soon enlarged to great gulleys by the 

 torrents of rain that fall. We may suppose that on the Saskatchewan, where there 

 is only a small quantity of rain, the winter's frost effects the same result, but 

 with this difference, that the successive landslips, remaining unmoved, at last form 

 such a gentle slope that vegetation can retain its hold, so that the rent is finally 

 represented by a symmetrical valley. 



