R. G. MCCONNELL — THE YUKON AND MACKENZIE BASINS. 541 



to Great Slave lake, coasted along this lake to its outlet, and then descended the 

 Mackenzie river through its whole length to the head of its delta at the mouth of 

 Peel river, lat. G7° 45' N. From this point the Rocky Mountains, here ahout four 

 thousand feet in height, were recrossed by the Peel river portage to the head waters 

 of the Porcupine, and the latter followed westward, through its long ramparts, to 

 its junction with the Yukon. Then bending again to the south, the Yukon was as- 

 cended to old Fort Selkirk, where connection was made with the line of exploration 

 traversed in the previous summer by Dr. G. M. Dawson. Of the rivers mentioned, 

 the Makcenzie only had been previously examined by a geologist, and that only in 

 a cursory manner and before the subject of glaciation had received much attention. 



"We shall commence the descriptive journey at Great Slave lake. This lake is sit- 

 uated upon the western margin of the Archean axis, and had originally the form of 

 a great cross, with one arm penetrating the crystalline schists, while two others 

 stretched north and south along the junction of these with the newer sedimentaries, 

 and the fourth extended itself over the flat-lying Devonian to the west. The southern 

 arm has now completely disappeared, and its bed is filled with a great alluvial deposit 

 of clays, false-bedded sands, and fine gravels, which have been brought down by 

 Slave river and through which its tortuous channel now winds. Not satisfied with 

 burying the southern arm, this river is now pushing its delta far out to sea, and 

 threatens at no distant day to inflict a similar fate on the whole eastern portion of the 

 lake. The time spent on Great Slave lake was not sufficient to enable me to form a 

 theory as to its origin which would have much value, but its peculiar shape, the 

 great depth of the Archean portion taken in connection with the comparatively low 

 elevation of the country which surrounds it, and the precipitous cliffs which border 

 the shores of this part in so many places, seem inexplicable by glacial agencies. It is 

 possible, however, that the western portion, which is much shallower and has low. 

 shelving shores, may have been excavated in part or altogether by a glacier forcing 

 its way out of a previously formed basin to the east. No very distinct groovings or 

 stria? were observed around the lake, but the hummocks of the roches moutonnees 

 gneissic surface of the country in the vicinity of Fort Rae have their major axes gen- 

 erally orientated in a direction about S. 30° W., or diagonally across the great 

 northern arm of the lake on which the fort is situated. Well defined glacial hum- 

 mocks carved out of massive dolomites were observed in one place on the western , 

 arm running in a westerly direction, or almost parallel to the general course "I this 

 portion of the lake. 



Great Slave lake, like the other great lakes to the south lying along the Archean 

 boundary, affords proof in the terraces surrounding it of former higher levels of it- 

 waters. Fragments of two lines of terraces were noticed in a number of places 

 around the western arm of the lake. The greatest elevation of these did not, how- 

 ever, exceed 30 feet above the present surface of the water. 



Hay river, which enters great Slave lake near its western end and drains the coun- 

 try to the southwest, has evidently had a history somewhat similar to that of the 

 Niagara; but it has not vet been thoroughly explored. In its lower part its valley 

 is carved out of a soft shaly terrane holding Hamilton fossils. Fifty miles above its 

 mouth a heavy band of cream-colored limestone overlying the shales crosses the river, 

 and a striking change is at once observed in the aspect of tin' valley. As we advance, 

 the valley contracts and becomes a gorge, so deep and narrow that its precipitous 

 walls, buttressed below by an embankment of fallen fragments, almost appear to over- 

 hang the stream, while the river, reduced in width in some parts to 100 feet, dashes 



LXXII— Bull. CJeol. Soi . \m., Vot. 1,1889. 



