SUPERFICIAL GEOLOGY OF THE CENTRAL REGION OF N. AMERICA. 603 



44. On the Superficial Geology of the Central Region of North 

 America. By George M. Dawson, Esq., Assoc. R.S.M., Geologist 

 H.M. North- American Boundary Commission. (Read June 23, 

 1875.) 



(Communicated by Dr. Bigsby, F.K.S., F.a.S.) 



[Plate XXXII.] 



Physical Geography of the Region. 



Where the great region of plain and prairie which occupies the 

 whole central part of Mexico and of the United States crosses 

 the forty-ninth parallel of latitude, which constitutes the political 

 boundary between the last-named country and British North Ame- 

 rica, it is included in longitude between the 96th and 114th meri- 

 dians. It narrows pretty rapidly northwards, chiefly by the 

 encroachment on it of its eastern border, but continues as a great 

 physical feature even to the shore of the Arctic ocean, where it ap- 

 pears to have a breadth of between 300 and 400 miles. North of 

 the North Saskatchewan river, however, it loses to a great extent its 

 prairie character, and, with the increasing moisture of the climate, 

 becomes thickly covered with coniferous forest. 



The eastern boundary of this interior continental plateau, north 

 of latitude forty-nine, is formed by the western slope of that old 

 crystalline nucleus of the continent, which extends north of the 

 St. Lawrence and the great Lakes from Labrador to the Lake of the 

 Woods, with a general east and west course, and then, turning sud- 

 denly at an angle of about 60° to its former general direction, runs 

 with a north-north-west course to the Arctic sea. This boundary, 

 though formed, wherever it has been carefully studied, in part of 

 less-metamorphosed rocks generally attributed to the Huronian, may 

 be called the Laurentian axis (see map, PI. XXXII.) In this part of 

 its course it is not of the nature of a mountain-range. It probably 

 does not attain a height of over 1500 to 2000 feet, and has an average 

 breadth of about 250 miles. It may rather be considered a great 

 rocky plateau ; and though it forms the division between the streams 

 running directly into Hudson's Bay and those flowing westward and 

 southward, the actual line of watershed has no determinate direction 

 on it, but follows a devious curve, which in one place (to the east of 

 the region now under consideration) approaches within twenty miles 

 of Lake Superior. Neither is it always a continuous barrier ; for 

 near the north end of Lake Winnipeg it is broken through by the 

 Nelson and Churchill rivers, the former of which carries across into 

 Hudson's Bay a great part of the drainage of the plains. 



To the west the plateau is bounded by the Rocky Mountains, 

 which rise abruptly from the elevated plain at their base, presenting 

 often to the east almost perpendicular walls of rock. They are 



