1854.] BIGSBY — GEOLOGY OF RAINY LAKE. 221 



The extensive shallow indent, called Black Bay, however, is based 

 on granular gneiss with white mica, and with small quantities of green- 

 stone-slate interleaved ; both having an E. by N. or E.N.E. strike 

 and northerly dip. 



The greater part of the interval of four miles between Point Logan 

 and Rainy River is occupied by syenite, in districts covered up 

 irregularly by morass. It is very dark, from the preponderance of 

 hornblende. It frequently contains isolated sheets of very micaceous 

 gneiss and dark greenstone-slate ; both with the usual E.N.E strike 

 and northerly dip, but not without one or two small deviations. 



At the commencement of Rainy River, on both banks, and for two 

 miles of the south shore of the lake, there is a large quantity of 

 untravelled debris of an Upper Silurian limestone, which is always 

 sharp-edged and slaty ; and now and then is planted into the earth 

 in such great square masses, that I am constrained to consider it 

 living rock, split into fragments by the intense cold of these regions. 



Containing the same fossils as the limestone of the lake of the woods, 

 I believe it to be of the same age*. It is browner and coarser in 

 texture. There is not much doubt but that it underlies most of 

 the bed of Rainy River, and is continued into the plains about the 

 Red River settlement. 



Remarks. — Whilst vast districts to the south and west are buried 

 under clays, sand, gravel, and boulders, to the depth occasionally of 

 .500 feet, there is about Rainy Lake but little loose debris ; the earth 

 or gravel banks being few, and seldom exceeding a few feet in thick- 

 nessf . "Whenever the land rises a little we have for the most part 

 bleached and naked rock for many square miles together. Some 

 northern erratic blocks there are, but not in such marvellous accu- 

 mulation as elsewhere encumbers the surface. 



The strikes in Rainy Lake, of which that to the E.N.E. may be 

 taken as normal or common, require a correction of 10° or 12° 

 westerly ; such being the amount of the magnetic variation easterly. 

 Thi? brings the strikes nearer to the N.E. 



The rock-formations of this lake are a continuation of those on the 

 south; — both towards the head-waters of the Mississippi, and the Fond 

 du Lac of Lake Superior ; the rocks themselves, their mutual relations, 

 and their strikes being similar. 



Dr. Norwood in his map of these parts of Minnesota (Atlas, Dale 

 Owen's Survey of Minnesota, &c.) covers the country with granite 

 and syenite, mingled with metamorphic slates, going N.E., as far as 

 enormous quantities of coarse and fine drift have permitted him to 

 observe. 



On the west of the Fond du Lac, and between the Mississippi and 

 Minnesota Rivers, Dr. Norwood found the strike to be N.N.E. ; but 

 whether this indicates a formation of a different era from that of 

 Rainy Lake and other neighbouring parts, or is due to local magnetic 

 variation, so changeable among these lakes, cannot at present be 

 settled. 



* Quart. Journ. Geol. Soc. vol. viii. p. 405. 



f See Dr. Bigsbv on Canadian Erratics; Quart. Journ. Geol, Soc. vol. vii. p. 215. 



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