﻿1851.] 



IGSBY ON CANADIAN ERRATICS. 



235 



St. Lawrence, seventy miles below Quebec, I only found two small 

 fragments of the inclined shale of the opposite shore. 



In the Lake of the Woods, any existing current goes northwards, 

 but it brings none of the innumerable loose masses of limestone of 

 the south division of the lake into the northern part ; but erratics 

 of the latter are in millions in the south division. I think the present 

 currents brought into Lake Erie only a small portion of the rocks of 

 Lake Huron, as we now find them. Instances of well-known erratics 

 having been shifted by the ice of winter, in lakes and streams, and on 

 the sea-coast, have been already published by Mr. Logan, Mr. Holmes, 

 myself, and others. 



Twenty miles south of La Ronde in Lake Nipissing, and half a 

 mile from the south shore, there is an example of one of those piles 

 of square travelled rocks, similar to those observed by Sir Roderick 

 Murchison on the rivers of Siberia*. There is another in Lake 

 Croche, S. Hudson's Bay, two in Lake Huron, and one on the river 

 Ottawa. That in Lake Nipissing consists of a large pell-mell heap 

 of gneiss-slabs, with edges as sharp, and surfaces as clean as if they 

 had been quarried yesterday for gravestones and flung down there. 

 These must have been left on shallows by the ice during a spring- 

 freshet. 



Fig. 2. — Profile of the west bank of the Ottawa River, below the 

 Tesouac River (or Mattawa Rive?*) ; showing the ice-borne debris 

 left after freshets. 



Fig. 2 represents a similar fact seen on the river Ottawa, a little 

 below the river Mattawa or Tesouac, and about 320 miles above the 

 St. Lawrence. It is a long line of naked square blocks, lying between 

 the limestone cliff, bounding the river in spring, and the ordinary bed 

 of the stream, — a deep hollow intervening. 



It has been supposed by the Professors Rogers, I think, that there 

 have been from the north two discharges of erratics under water, 

 which were separated by a period of repose. I have no data for ap- 

 plying this idea to the Canadas. 



The Imbedded Detritus. Beds and Terraces. — The Canadas, in 

 common with all the western and northern parts of the United States, 

 are mapped out by irregular concentric rings of terraces and ridges, 

 sometimes hundreds of miles in circuit, which inclose the beds (with 

 or without water) of lakes and ponds more or less closely. The mouths 



* The Geology of Russia in Europe and the Ural Mountains. 



R 2 



