610 G. M. DAWSON ON THE STJPEKFICTAL GEOLOGY OF 



has given an account of the geology of the Lake of the Woods and of 

 the distribution and origin of the erratics there, in former volumes of 

 the Journal of this Society (Quart. Journ. Geol. Soc. 1851 and 1852.) 

 Its shores and islands are covered with boulders, most of which can 

 be traced to outcrops of similar rocks not far to the north-east ; but 

 here too a considerable quantity of limestone is found. It is not 

 generally in such large fragments as the metamorphic rocks, and is 

 often seen in small pebbles only, but occurs in some places in great 

 profusion. The limestone drift is entirely confined to the southern 

 and western shores of the lake ; and its origin is a question of some 

 difficulty. No similar rock is known to exist to the north-east, 

 unless the limestones of the shores of Hudson's Bay are of this 

 character. Limestone is known to occur on the western side of the 

 Laurentian axis 50 miles further north-west, and beyond that point 

 in great abundance. The limestone there found, however, is of Lower 

 Silurian and Devonian age, while the fossils in some, at least, of 

 the erratics prove them to be "Upper Silurian. A south-eastern 

 drift of floating ice may account for some of the specimens ; but I am 

 inclined to believe, with Dr. Bigsby, that an outcrop of Upper Silu- 

 rian is concealed by the drift- deposits along the base of the Lauren- 

 tian in the Lake-of-the-Woods region. 



Lowest Prairie-Level and Valley of the Red Uiver. 



Descending the western side of the drift plateau of Northern 

 Minnesota, we enter the valley of the Red River (PI. XXXII., b) ; by 

 which term I mean to express not the whole drainage-area of the stream 

 in a strict geographical sense, but the well-defined and comparatively 

 narrow trough holding the main stream, and here constituting the first 

 prairie-level, which is bounded westward by the front of the second 

 prairie -steppe. This trough runs nearly due north and south, and, 

 from the south shore of Lake Winnipeg to the source of the Red 

 River in Lake Traverse, is 315 miles in length. It does not end 

 here, however, but passes by a continuous gap, never more than 

 690 feet above the sea-level, to the source of the Minnesota River, a 

 tributary of the Mississippi. On the boundary-line the valley is 46 

 miles wide, and it narrows very gradually southwards. The floor 

 of the valley, though it slopes upwards towards the sides, does so 

 at so small an angle as to be quite imperceptible to the eye. It 

 presents an appearance of perfect horizontality, and is perhaps the 

 most absolutely level prairie-region of America. Looking down, 

 towards evening, through one of the breaches in the edge of the 

 western escarpment, it requires little imagination to suppose that 

 the bluish level expanse is that of the sea ; and, indeed, the whole of 

 this valley must, at a time geologically modern, have been occupied 

 by a great lake, the fine silty deposits of which now form its level 

 floor. On examining these deposits they are found to be arranged 

 in thin horizontal beds, which together constitute a great thickness, 

 and rest upon till or boulder-clay. Some of the layers immediately 

 overlying the till may correspond with those already described in the 



