20 ‘ Mackenzie Lowland. The lowland portion of the Great Central plain of the Mackenzie basin commences in the lower part of Peace river and the extreme western end of lake Athabaska, whence it extends as a narrow band down the valley of Slave river to Great Slave lake. Its eastern boundary here as well as north of Great’ Slave lake is the border of the Laurentian plateau. It embraces the basin of the western end of Great Slave lake and continues thence down the valley of Mackenzie river to the Arctic coast, its eastern boundary being as far as known the Laurentian plateau, and its western Mackenzie mountains and the escarpment of the Peel plateau. A re-entrant of the lowland extends up the valley of Liard river as far at least as Fort Liard. . The limits of the lowland on the south are well defined and marked by a distinct topographic break at the escarpment of the Alberta plateau. North of Liard river the lowland reaches on the west to the base of Mackenzie mountains which are here not bordered by an elevated plateau belt. North of Nahanni river, however, the long narrow ridge of Franklin mountains divides the lowland into two parts by separating a strip 20 to SO miles in width, through which the Mackenzie flows, from the main portion of the lowland lying in the drainage basin of Great Bear lake. Franklin mountains are believed to die out about lati- tude 66 degrees, and from this line northward the lowland is bounded on the west by the escarpment of the Peel plateau and farther north by Richardson mountains, whereas on the east its limits are unknown though it probably extends across the height of land into the watershed of Anderson river. So far as it is possible to determine from our imperfect knowledge of the eastern portions of the lowland there is no well-defined and continuous break between the lowland and the Laurentian plateau such as occurs in the west, but the plateau, as a rule, dips gently westward under the overlapping edges of the flat-lying sedimentary strata which constitute the bedrock of the lowland. The elevation of the lowland at the west end of Athabaska lake is about 700 feet above the sea and the slope of the surface from that point to the Arctic averages about 8 inches to the mile and is so gradual that from Fort Smith to the Arctic ocean, a distance of over 1,300 miles by the river, steamers with a draft of 5 feet are able to run without encountering any obstructions from rapids ormallgs +4 hua! The surface as a rule does not stand much above the main water courses or the great lakes and the banks of these are rarely more than 200 feet in height. The great bodies of water, Great Slave and Great Bear lakes, occupy areas of depression in the surface of the lowland, and another long narrow depression, not, however, now filled with water, lies along the base of Mackenzie mountains from Fort Liard to the mouth of North Nahanni river. Rising above the general level of the lowland are a number of hills and mountain ranges varying in height from a few hundreds of feet to about 4,000 feet. Such ranges are: Horn mountain, northwest of Providence; Franklin mountains, between the Mackenzie and Great Bear lake; Grizzly Bear mountain and Scented Grass hills on the shores of Great Bear lake; Reindeer hills, east of the delta; and a-number of other short ranges or isolated hills. : These ranges are the only features that rélieve the monotony of an otherwise flat plain which holds on its forested surface innumerable lakes and muskegs, and through which the smaller streams meander in shallow valleys. Only in the 1McConnell, ‘R. G., Geol. Surv., Can., vol. IV, 1888-89, pp. 57 D and 89 D.