﻿1851.] 



BIGSBY ON CANADIAN ERRATICS. 



221 



boulders, one-half of which are trap, the remainder being brown and 

 greenish amygdaloids, brown and green porphyries, granite, gneiss, 

 jasper-pudding-stone, greenstone-conglomerate, hornblende-rock, and 

 sandstone,— all directly traceable to the north and east*. 



St. Mary's River, on its left side, is faced with coarse ferruginous 

 sand-banks (derived from the sub-rock), with trap and sandstone 

 blocks on Pine Point. 



The Rapids or Falls of St. Mary are surrounded with great collec- 

 tions of travelled rocks, both on the Canadian and the American sides 

 of the river. The chief are gneiss, greenstone, granite, jasper-pudding- 

 stone, mica-slate, greenstone, porphyry, conglomerate, and angular 

 masses of the sub-rock (Potsdam sandstone). I saw one piece of 

 the La Cloche crystalline quartz-rock. The small bays and coves 

 and the river itself, above the Falls, are crowded with these erratics. 



Bedded Detritus of the N. Shore. — Although it be true that much 

 of this north border presents only precipices, rugged steeps, and 

 dome-shaped hills, varying in elevation from 100 to 1400 feet above 

 the lake, swept bare by arctic winds, or washed by melting snow, 

 nevertheless in their intervals we find arenaceous and argillaceous 

 deposits in large areas for 100 miles inland (according to our present 

 knowledge), and bearing thin forests of pine, birch, and poplar. 



These deposits also face the lake in the shape of bare earth-banks 

 and terraces. They are all the product of the lake when standing 

 at higher levels ; and are local drift, deposited most commonly in 

 times of disturbance, but not always, as we find them stratified in 

 Michipicoton, Huggewong, and Batchewine Bays. Most of the 

 materials are from fixed rocks close at hand. 



The first set of ledges I saw in travelling eastwards was in the 

 Mammelles Isles, consisting principally of porphyry shingle. They 

 are low, not exceeding twenty feet altogether, although from one to 

 six in number passing inland for one-third to two-thirds of a mile, 

 and are dotted with trap-boulders. 



These are formed by the tempests of the present day. Not so, 

 however, the great beds about the mouth of the picturesque Black 

 River, opposite the Slate Islands. At the lake-side these earth- 

 banks are 330 feet high (Logan), sloping downwards in the rear, and 

 consist of very small angular and round bits of granite, trap, and 

 quartz, imbedded in a coarse dark brown sand, pressed hard together. 

 The granite is derived from the disintegration of the hills of that 

 rock hard by, and the other rocks are in place nigh at hand. 



This deposit forms a plain, naked save for a few wretched pines 

 and, here and there, a block of brown quartz-rock or gneiss, derived 

 from the north. It extends five or six miles into the interior to a 

 line of high bare hills. Along the lake-side it extends east and west 

 as far as the eye can see from a considerable elevation ; and it is in- 

 tersected by the Black River, which has three terraces on each bank. 

 In the middle of the plain I found a pair of deer's antlers lying on 

 the ground. Near the lake, one mile and a half west from the 



* Mr. Logan has seen specimens of jasper-pudding-stone found in situ in Gou- 

 lais Bay. 



