PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 419 
protected by a calcareous sheath, algae could be preserved only 
under exceptional conditions. We have, then, no definite knowl- 
edge of the development of attached algae in Ordovician and Silu- 
rian seas. In the strata in question, indisputable fucoids have 
been obtained only from the Cat Head and Stony Mountain beds, 
which are nowhere mottled, and are uniformly dolomitized. They 
are found as imperfect markings, and generally as raised impres- 
sions, and have been referred by Whiteaves to five distinct species 
of Chondrites. Numerous unidentified markings on the bedding 
planes of the limestones have also been ascribed to fucoids. 
If one may judge from the amount of carbonaceous material 
in rocks prior to Silurian times, it is probable that the algae had 
already attained a widespread development. The conditions 
which would give rise to dolomitization from plant forms which are 
so widespread in later seas must of course have been exceptional, 
and the chemical constitution of the seas of the period may provide 
the best explanation for the phenomenon. The evidence that has 
elsewhere been collected? goes to show that the percentage of mag- 
nesium salts in the early Palaeozoic seas was distinctly higher 
than in the ocean today. In the seas in which the Lower and Upper 
Mottled were laid down it was not sufficiently high to cause dolomi- 
tization in the sense that actual crystals with the optical properties 
of dolomite were produced; but the addition of magnesium salts 
from the decomposing algae was all that was required to start the 
process. Changing physical conditions—probably a shallowing of 
the sea—increased the Mg content at the time when the Cat 
Head and Stony Mountain formations respectively were being 
deposited, and in these formations a uniform, though by no means 
complete, dolomitization was effected. The algae may have drifted 
seaward from the rocky shores, and may have been fairly rapidly 
silted over. It is worthy of note that the presence of unicellular 
algae of the plankton has been confirmed in rocks of similar age 
in Wisconsin, and has been referred to, as already indicated, in 
the limestones of the upper Cambrian in the northwest Highlands 
of Scotland. The ‘‘oil rock” in the Galena of Wisconsin is found 
* Geological Survey of Canada, Palaeozoic Fossils, III, Pts. 1 and 2. 
2 Steidtmann, Joc. cit. 
