﻿1851.] 



BIGSBY ON CANADIAN ERRATICS. 



219 



on to remark in some detail on the loose materials of the north shore 

 of Lake Superior, — a body of water occupying a crescent-shaped 

 trough, 1700 miles round, 420 miles long, by 163 miles in its greatest 

 breadth. Prof. Agassiz is of opinion that it owes much of its present 

 size and form to successive eruptions of basalt. Its surface is 623 

 feet above the sea*. By far the greater part of its debris is strictly 

 local ; a second part belongs to a distant part of the lake, ranging 

 from two to eighty miles ; while a third, the distant or foreign erra- 

 tics, comes from the granitoid, trappose, and Silurian regions of the 

 north ; much of it from the first 400 miles north of Lake Superior. 



These large, somewhat angular, foreign boulders do not often 

 meet the eye, because much of the north shore of this lake is rock- 

 bound, its beaches narrow, and its waters deepen rapidly, while the 

 adjacent country is steep, and its confined valleys are loaded with 

 moss and other vegetation. 



The lip or margin of the south shore, on the contrary, is compara- 

 tively low, and the region around rather level, some of its greatest 

 elevations at the water-side being high sand-hills, formed by the pre- 

 vailing winds from the friable sandstone of the south-east of the lake. 



The debris which can be traced home on the north shore of Lake 

 Superior is in great quantity, and is either large (blocks from 1 inch 

 to 15 feet long) and lying naked more or less profusely on the 

 beaches, or it is sand and grit, forming horizontal deposits of great 

 thickness, which border the lake and its tributary streams in the shape 

 of terraces, and which enwrap the neighbouring hills in level plains, 

 sometimes small, at others extending many miles in every direction. 



We will first speak of the detritus at the water's edge ; — beginning 

 at the Grand Portage, the most westerly point of Lake Superior with 

 which I am acquainted, and 442 miles from the Straits of St. Mary 

 leading into Lake Huron. 



At the Grand Portage the beach is strewn with various traps, 

 amygdaloids, red and brown argillaceous porphyries, masses of baked 

 clay, or coarse jasper, red and white sandstone (the last traceable to 

 a locality six miles distant to the N.E.), clay-slate with veins of quartz 

 and amethyst, chert, and some limestone. Fixed rocks exactly 

 similar to these are found at Nipigon, &c, on the E.N.E. ; but the 

 detritus may possibly have come from Isle Royale. It may be here 

 remarked, that these boulders have travelled from the N.E., contrary 

 to the prevailing winds and to the existing lake-current. The iron- 

 bound coast from the Grand Portage, for sixty miles eastwards, to 

 near the Mammelles, for the most part merely exhibits sharp-edged 

 fragments of the trappose sub-rock. 



The beaches of the numerous isles of the Mammelles are loaded 

 with large traps, indurated red sandstone, and a pale limestone (Silu- 

 rian) rolled into balls of from 3 to 8 inches in diameter. All this 

 detritus, excepting the limestone, is from the vicinity, — from the north 

 or north-east ; limestone is not known to occur in situ on the north 

 shore, except at Thunder Head. 



* The true height of Lake Superior is 600 feet above the sea, according to Mr. 

 Logan. 



Q 2 



