1855.] ISBISTER — NORTH AMERICA, ETC. 503 



visible. The small elevation of the chain may be inferred from an 

 examination of the Map,' which shows that it is crossed by several 

 rivers that rise in the Rocky Mountains, the most considerable of 

 which are the Churchill, and the Saskatchewan or Nelson Rivers. 

 These great streams have, for many hundred miles from their origin, 

 the ordinary appearance of rivers in being bounded by continuous 

 parallel banks, but on entering the primitive district, they present 

 chains of lake-like dilatations, which are full of islands and have 

 a very irregular outline. Many of the numerous arms of these ex- 

 pansions wind for miles through the neighbouring country, and the 

 whole district bears a striking resemblance, in the manner in which 

 it is intersected by water, to the coast of Norway and the adjoining 

 part of Sweden. The successive dilatations of the rivers have scarcely 

 any current, but are connected with each other by one or more straits, 

 in which the water-course is more or less obstructed by rocks, and 

 the stream is very turbulent and rapid. The most prevalent rock in 

 the chain is gneiss ; but there are also granite and mica-slate, toge- 

 ther with numerous beds of amphibolic rocks." 



The entire length of this remarkable plateau, from Lake Superior 

 to its termination on the Arctic Sea, may be estimated at somewhat 

 more than 1 500 miles. Such an enormous extension of crystalline and 

 eruptive rocks, nowhere assuming the character of a mountain district, 

 is a remarkable example of the tranquil operation of an upheaving 

 force exerted over an immense area, yet with a limited and regulated 

 intensity, and a constancy of direction which render it worthy of atten- 

 tion, not only as a striking geological phaenomenon, but as serving, 

 perhaps, to throw some light on the dynamical conditions under which 

 those vast sedimentary deposits which have excited the astonishment 

 of geologists in America from their unparalleled extension have been 

 originally upheaved. 



It may be mentioned also as another remarkable circumstance in 

 connexion with this granitic tract, that it is along its western mar- 

 gin, in the line of its junction with the limestones and other second- 

 ary deposits which extend between it and the Rocky Mountains, that 

 all the great lakes of America are found. If we regard Lake Erie 

 and Lake Michigan as expansions respectively of Lake Ontario and 

 Lake Huron (being evidently component parts of the same lake- 

 basins), we shall find the following series of great lakes — Lake On- 

 tario, Lake Huron, Lake Superior, Lake Winipeg, Athabasca Lake, 

 Great Slave Lake, Marten Lake, and Great Bear Lake, succeeding 

 one another in a N.N.W. direction along the line of fracture, and in- 

 variably bounded to a greater or less extent on one side (generally 

 the northern or eastern) by crystalline rocks, and on the opposite side 

 by limestones and other secondary formations ; the northern coast-line 

 being moreover indented nearly in the same general bearing by Coro- 

 nation Gulf, where, as already stated, the line of crystalline rocks 

 terminates. It is to be observed, however, that the rivers connecting 

 these lakes run generally wholly in one formation or in the other. 



Silurian Basin of Hudson's Bay. — The granitic tract just de- 



