THE CENTRAL REGION OF NORTH AMERICA. 621 



with the direction required by glaciatioh, according to Mr. Selwyn's 

 measurements it cannot be over 1400 feet. The height of land in 

 other parts of the Laurentian region is very uniformly between about 

 1600 and 1200 feet. Allowing, then, 1600 feet as a maximum for the 

 region north-east of the Lake of the Woods, and taking into account 

 the height of that lake and the distance, the general slope is not 

 greater than about 3 feet per mile — an estimate agreeing closely with 

 the last, which is for a smaller area and obtained in a different 

 way. This slope cannot be considered sufficient to impel a glacier 

 over a rocky surface which Sir William Logan has well character- 

 ized as " mamillated," unless the glacier be a confluent one pressed 

 outwards mainly by its own weight and mass. 



Such a glacier, I conceive, must have occupied the Laurentian 

 highlands ; and from its wall-like front were detached the icebergs 

 which strewed ths debris over the then submerged plains, and gave 

 rise to the various monuments of its action now found there. 



The sea, or a body of water in communication with it, which may 

 have been during the first stages of the depression partly or almost 

 entirely fresh, crept slowly upward and spread westward across the 

 plains, carrying with it icebergs from the east and north. During 

 its progress most of the features of the glacial deposits were im- 

 pressed. In the section described at Long River we find evidence 

 of shallow current-deposited banks of local material, afterwards, 

 with deepening water, planed off by heavy ice depositing travelled 

 boulders. 



The sea reaching the edge of the slope constituting the front of 

 the highest prairie-level, the deposition of the Coteau began, and 

 must have kept pace with the increasing depth of the water and 

 prevented the action of heavy ice on the front of the Tertiary 

 plateau. The water may also have been too much encumbered with 

 ice to allow the formation of heavy waves. 



The isolated drift highlands of the second plateau, including the 

 Touchwood Hills, Moose Mountain and Turtle Mountain, must also at 

 this time have been formed. With regard to the two former, I do not 

 know whether there is any preglacial nucleus round which drift- 

 bearing icebergs may have gathered. There is no reason to suppose 

 that Turtle Mountain had any such predisposing cause ; but it would 

 appear that a shoal once formed, by currents or otherwise, must have 

 been perpetuated and built up in an increasing ratio by the ground- 

 ing of the floating ice. 



The Eocky Mountains were probably also at this time covered 

 with descending glaciers ; but these would appear to have been 

 smaller than those of the Laurentian axis, as might, indeed, be pre- 

 supposed from their position and comparatively small gathering- 

 surface. The sea, when it reached their base, received from them 

 smaller icebergs ; and by these and the shore-ice the quartzite- drift 

 deposits appear to have been spread. That this material should have 

 travelled in an opposite direction to the greater mass of the drift is 

 not strange ; for while the larger eastern and northern icebergs may 

 have moved with the deeper currents, the smaller western ice may 



