516 Reports awl Proceedings — 



presents an appearance of perfect horizontality. The soil consists 

 of fine silty deposits, arranged in thin horizontal beds, resting on 

 till or boulder-clay. Stones were exceedingly rare. The western 

 escarpment was terraced and covered with boulders. It is therefore 

 probable that this prairie is the bed of a preglacial lake. 



The Second Prairie Plateau is thickly covered with drift deposits, 

 which consist in great part of local debris, derived from the under- 

 lying soft formations, mixed with a considerable quantity of trans- 

 ported material, especially in the upper layers. Large erratics are 

 in places abundant ; they consist mainly of Laurentian rocks, but 

 Silurian limestone also abounds. The following is the per-centage 

 of the boulders from the different formations present in the drift : — 

 Laurentian 28 49, Huronian 9-71, Limestone 54-01, Quartzite Drift 

 144. The last is derived from the Eocky Mountains, the other 

 three from the Laurentian axis. There are also on the surface of 

 this plateau some remarkable elevated regions, apparently entirely 

 composed of accumulated drift materials. 



Edge of the Third Prairie Plateau, or the Missouri Coteau, is a mass 

 of glacial debris and travelled blocks averaging from 30 to 40 miles 

 in breadth, and extending diagonally across the country for a 

 distance of about 800 miles. 



Third or Highest Plateau. — There is a marked change in the 

 drift on this plateau, the quartzite drift of the Eocky Mountains pre- 

 ponderating, seldom showing much glaciation. Its general cha- 

 racter may be seen from the following per-centage of its composition : — 

 Laurentian 27-05, Huronian ? Limestone 15-84, Quartzite drift 52-10. 

 Some of the lower parts of this steppe show thick deposits of true 

 till with well-glaciated stones, both from the mountains and the east, 

 and debris from underlying Tertiary beds, all in a hard yellowish 

 sandy matrix. On the higher prairie sloping up to the Eocky 

 Mountains the drift is entirely composed of material derived from 

 them. 



The Boclcy Mountains themselves show abundant traces of glacia- 

 tion. Nearly all the valleys hold remnants of moraines, some of 

 them still very perfect. The harder rocks show the usual rounded 

 forms, but striation was only observed in a single locality, and there 

 coincided with the main direction of the valley. The longer valleys 

 generally terminate in cirques, with almost perpendicular rock- walls, 

 and containing small but deep lakes. 



State of the interior region of the continent previous to the Glacial 

 Period. — The author considers that previous to the glacial epoch the 

 country was at about its present elevation, and that its main physical 

 features and river-drainage were already outlined. Subaerial de- 

 nudation had been in operation for a vast period of time, and an 

 enormous mass of Tertiary and Cretaceous strata removed. 



Mode of Glaciation and Formation of the Drift Deposits. — The 

 author did not find any evidence rendering the siipposition of a 

 great northern ice-cap necessary, but suggests that local glaciers on 

 the Laurentian axis furnished icebergs laden with boulders, which 

 were floated across the then submerged prairies towards the Eocky 

 Mountains. 



