Beyond the Mississippi the line follows approximately the course of the Missouri River across Missouri, and continues westward to the vicinity of Manhattan, in Kansas, where it turns northward, keeping about a hundred miles west of the Missouri River, through eastern Kansas and Nebraska, and striking the river near the mouth of the Niobrara, in South Dakota. From there the line follows approximately the course of the Missouri River to the vicinity of Fort Benton, in northwestern Montana, where the line again bears more northward, running into British America. It is still in dispute whether the ice extended from the eastern centre far enough west to join the ice-movement from the Rocky Mountain plateau. Dr. George M. Dawson[BC] is of the opinion that it did not, but that there was a belt of a hundred miles or more, east of the Rocky Mountains, which was never covered by true glacial ice. Mr. Upham[BD] is equally confident that the two ice-movements became confluent, and united upon the western plateau of Manitoba. The opportunity for such a difference of opinion arises in the difficulty sometimes encountered of distinguishing between a direct glacial deposit and a deposit taking place in water containing boulder-laden icebergs. Where Mr. Upham supposes the ice-fields of the east and of the west to have been confluent in western Manitoba, Dr. Dawson supposes there was an extensive subsidence of the land sufficient to admit the waters of the ocean. Leaving this question for the present undetermined, we will now rapidly summarise the glacial phenomena west of the third meridian from Washington (which corresponds nearly with the western boundary of Pennsylvania), and east of the Rocky Mountains. [Footnote BC: Transactions of the Royal Society of Canada, vol. viii, sec. iv, pp. 54-74.] [Footnote BD: American Geologist, vol. vi, September, 1890; Bulletin of the Geological Society of America, vol. ii, pp. 243-276.]