PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 
IN MANITOBA 
R. C. WALLACE 
University of Manitoba, Winnipeg 
THE ORDOVICIAN IN MANITOBA 
The Ordovician in Manitoba, as determined by Dowling,’ 
consists of the following series in ascending order: (1) The Winni- 
peg sandstones, directly overlying the eroded surface of the pre- 
Cambrian. This division consists of beds of soft, often very 
white, friable sandstones, about 1oo ft. thick, containing only a 
few fossils. The St. Peter sandstone of Minnesota is similar in 
petrological aspect, but the absence of true Chazy fossils in the 
Winnipeg sandstone procludes definite correlation with the St. 
Peter sandstone. The fossils rather suggest upper beds of the 
Black River, but this formation is in Minnesota represented by 
shales. (2) The Lower Mottled limestone, exposed along the west 
side of Lake Winnipeg, and on some of the islands. This forma- 
tion is about 70 ft. thick. The character of the limestone is very 
similar to that to be described in detail as the Upper Mottled lime- 
stone. It is mottled buff and greyish white, is highly fossiliferous, 
and shows what are probably fucoidal tracings on the bedding 
planes. The upper beds are highly charged with siliceous material. 
(3) The Cat Head limestone, yellow dolomites, even grained and as 
a rule fine grained, containing cherty concretions which in many 
instances exceed a foot in diameter. The beds have been esti- 
mated to be 68 ft. thick. The large cephalopods which are so 
common in the Lower Mottled are wanting in the Cat Head series. 
(4) The Upper Mottled limestone, exposed at several points on the 
western shores of the northern extension of Lake Winnipeg; also 
along the Red River, and at the time of Dowling’s work more 
particularly at East Selkirk. Today the limestone quarries of 
Tyndall provide very good sections of this limestone. The thick- 
t Geological Survey of Canada, Annual Report, 1900. 
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