412 R. C. WALLACE 
for odliths and corals. In the case of vein dolomitization, calci- 
fication usually precedes dolomitization, the softer, abraded 
material being first attacked by water and recrystallized, while 
the Mg-bearing waters subsequently attack what has not been 
affected. Thus the parts that are affected by the Mg-bearing waters 
in subsequent dolomitization are just such as contemporaneous 
dolomitization would not affect—odliths, corals, etc., the hardest 
parts in ordinary sediments but the softest in the recalcified rock. 
Judged by Dixon’s tests, the pseudobrecciation of the Manitoba 
limestones is contemporaneous. There is no secondary calcifica- 
tion, the harder shells of corals, brachiopods, etc., have not been 
attacked in preference to the matrix, and the hematite is not 
included in the dolomite crystals. It seems difficult to imagine how 
subsequent dolomitization could affect a limestone in such a way 
that throughout a thickness of almost a hundred feet a uniform 
mottling should be produced, the affected areas being unconnected 
vertically. Leonard* seems to favor the view that the dolomitiza- 
tion in Clayton County is a subsequent phenomenon, in the sense 
that it was probably contemporaneous with the dolomitization of 
the Galena. This was inferred from‘ the fact that the mottled 
limestones were found separating the fully dolomitized Galena 
from the underlying undolomitized Trenton in all cases except 
where impervious shales formed a sharp boundary line between the 
uniformly dolomitized and the non-dolomitized strata. The actual 
conditions are, however, different in Manitoba. Both the Lower 
Mottled and Upper Mottled limestones are overlaid by more com- 
pletely dolomitized horizons—the Cat Head and the Stony Moun- 
tain formations—but a gradation from a mottled stone to a grey 
undolomitized limestone has nowhere been observed; nor can it 
be said that there is in any measure a gradation from a sparingly 
dolomitized variety by regular stages into a typical dolomite. 
The proportion of darker material is as great at the base of a section 
of mottled limestone 97 ft. thick as it is at the top. 
CONTEMPORANEOUS DOLOMITIZATION 
The evidence goes to show that the dolomitization in this area 
took place more probably as a practically contemporaneous 
t Loc. cit. 
