On the Geology of the Northwestern Regions of America. 323 
edge of the granitic plateau, which would give an average breadth 
of about 200 miles for the formation in this part. The boat- 
route from Lake Winnipeg to York Factory crosses the limestone 
belt at right angles to its course at Rock Portage, and its breadth 
is there found to diminish to less than 100 miles. The average 
width of the formation may perhaps be estimated at about 150 
miles. 
The mineral structure of the rocks forming the northern shores 
of America has been so fully and minutely investigated and de- 
scribed by Prof. Jameson, Mr Konig, Dr. Fitton, and Sir John 
Richardson, that I shall here, as well as in the notices of the 
other formations of this territory, confine myself exclusively to 
the examination of their organic remains, referring the reader for 
every necessary information on the mineralogical character of the 
rocks in which they are found to the valuable publications of 
those authors. 
Silurian Basin of Lake Winipeg.—To the westward of the 
teau of crystalline rocks, and following its course for a consid- 
erable distance northward, lies an extensive deposit of horizontal 
limestone, underlying the wide prairie country which extends 
towards the Rocky Mountains. Lake Winipeg, which is situated 
on the line of junction of the two formations, is a long and 
harrow sheet of water, 230 geographical miles long, and about 
0 wide; and with its associated lakes (Moose Lake, Muddy © 
‘ake, Winepegoos, and Manitoba Lakes), receives, through its 
affiuents—the Saskatchewan, the Red River, and other streams 
—a wide extent of prairie drainage. The commercial route from 
ke Superior up to this point lies almost wholly within the 
granitic tract, touching on Silurian deposits only at the mouth of 
iny River and at one of the southwestern arms of the Lake of 
the Woods, where Dr. Bigsby has detected a few organic remains 
indicative of the Upper Silurian formation.* The Wini 
flows wholly within the granitic district, and has the lake-like 
dilatations and other characteristics of the streams which traverse 
the crystalline tract. When we descend to Lake Winipeg, we 
lake being mostly excavated in the limestone. The granite and 
Sneiss which form the east shore of Lake Winipeg strike off at 
its NE corner, and passing to the north of Moose Lake, go on to 
* The followin: list is given by Dr. Bigsby: a small Phacops, small Orthocerata, 
minute Enerinital edtltiin. © Vcgoaties etbandica, Cyathophyllum, Murchisonia, 
Pentamerus Knightii, Leptena, Avicula, Atrypa, and Spirifer. Quart. Journ. Geol. 
Soc, vol viii, p. 405. 
i 
