420 R. C. WALLACE 
on microscopical examination to contain very numerous oval yellow 
bodies, which are interpreted by White’ to be unicellular gelosic 
algae, probably comparable to the living Protococcales. The 
precipitation of the well-known lead and zinc deposits of this 
formation is attributed by Bain? to the reducing action of the hydro- 
carbons, and probably to hydrogen sulphide, resulting from the 
partial decomposition of the algae. Whether in the Manitoba 
limestones the mottling may be accounted for in part by unicel- 
ular algae settling in local depressions on the sea floor—caused for 
instance by the burrowings of annelids or other animals in the silt— 
the writer is not prepared to say. After reviewing, however, the 
hypotheses which may be advanced to account for a phenomenon 
the origin of which it is difficult indeed precisely to define, he is 
compelled to conclude that the evidence is strongly in favor of 
the theory that the decomposition of algae has been primarily 
responsible for the local dolomitization which is so marked a feature 
of these limestones. 
SUMMARY 
The irregular mottling which is a characteristic feature of two 
horizons of the Ordovician limestones in Manitoba is due to the 
presence of certain dolomitic areas in the limestone. The color 
contrast is caused by hematite and limonite filling the interstices 
between the dolomite crystals, rendering the affected areas much 
darker than the non-dolomitic. 
The apparently brecciated structure is not truly clastic. The 
darker areas have been dolomitized im situ by Mg-bearing waters, 
working from the center outward. 
Chemical analyses show that the iron salts have been carried 
in the waters which affected the transformation, and the iron 
minerals are not, as might be supposed from microscopical investi- 
gation, the result of the oxidation, when recrystallization took 
place, of ferrous carbonate held isomorphously in the calcitic 
material of the limestone. 
The evidence in the field and laboratory is sufficiently convin- 
cing to lead one to conclude that the dolomitization is not a subse- 
1 Wisconsin Geol. and Nat. Hist. Soc., XIX, 26. 
2 [bid., 142. 
