58 Dr J.J. Bigsby on the 



is a consistent and closely connected whole, forming a beautiful and 

 easily read example of geological action in moulding the surface of 

 our globe. 



The lake may best be presented at once to the mind as a trough 

 or basin of Cambrian (or Silurian) sandstone, surrounded and 

 framed, as it were, by two orders of rocks, in the form of irregular 

 and imperfect zones ; the inner consisting of trap, with its conglo- 

 merates ; and the outer, of metamorphic, flanking igneous rocks. 



1. The Metamorphic rocks, with the exception of quartzite and 

 jasper, are the oldest in the lake, and support great sheets of the 

 above-mentioned sandstone unconformably ; all these rocks being 

 upheaved and altered by the intrusion of igneous rocks in instances 

 innumerable. This group of rocks is entirely destitute of the traces 

 of animal life. 



The country they occupy on the south shore, with a general 

 NNW. dip, may be best described as a rough table land of the 

 various slates, out of which short hills of granite, gneiss, trap, &c, 

 emerge in great numbers, with an almost constant east and west 

 direction. 



On the east and north shores the metamorphic rocks have a W. 

 and WSW. strike, when visible. The slates of the north side of 

 Michipicoton Bay run WNW., NW„ and N. 



The jasper and quartzite are merely altered sandstone and there- 

 fore younger than the other rocks of this group. 



2. The Aqueous Rocks. — The youngest of these is calciferous 

 sandstone. It exists as a broad band on the south-east shore, resting 

 on the sandstone soon to be noticed. It is highly magnesian and 

 siliceous in parts. A patch of it in Grand Island contains shells. 

 (Logan.) 



The Cambrian Sandstone seems to be the floor or basement of 

 nearly all the lake, for the following reasons : — 



1. Wherever it occurs, whether in immense sheets on the east 

 and south shore, or in smaller areas on the north coast, it 

 invariably dips towards the centre of the lake. 



2. It can be recognised, paving the lake for some miles from 

 the main in many places. 



3. The soundings of Captain Bayfield exhibit, for large spaces, 

 the uniformity of level to be expected from the presence of 

 horizontal strata. 



4. Because it constitutes Caribou Island, 40 miles from the 

 nearest main land. 



This sandstone is very ancient ; and is supposed by Mr Logan 

 to be Cambrian on the north shore, and lower Silurian on the south — 

 a supposition, the latter clause of which, though extremely probable, 

 is not yet established. 



It has no fossils ; but its ripple marks, impressions of rain-drops, 

 a n<l sun-cracks, are plentiful and perfect. 



