On the Superficial Geology of the Central Region of North America

Abstract

W here 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 America, it is included in longitude between the 96th and 114th meridians. 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 appears 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 suddenly 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, Pl. XXXII.)In this part of its course it is not of the nature of a mountain-range. It probably does not attain a