PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 421 
quent phenomenon—due to the percolation of Mg-bearing waters 
from above through considerable thicknesses of limestone along 
lines of weakness, but took place as a practically contemporaneous 
process with the formation of the limestone, in the upper layers 
of the calcareous ooze of the sea bottom. 
A limestone may undergo uniform dolomitization when the 
percentage of Mg ions in the sea water affecting it is much smaller 
than the chemical equations usually taken to represent the process 
would demand. The percentage of Mg ions in the seas in which 
the Lower and Upper Mottled limestones in Manitoba were laid 
down was probably considerably higher than that in the sea today, 
but Jower than that necessary to cause dolomitization such as the 
darker areas have undergone. 
Under such conditions three factors would tend to produce 
local dolomitization: (1) local rise in temperature, (2) presence of 
aragonite in the calcareous ooze, (3) a greater percentage of Mg 
ions in the water permeating certain parts of the hardening ooze. 
The Jast factor is the preponderating one in the area under investi- 
gation. 
Three suggestions are considered as possible explanations of the 
irregular dolomitization: (1) sea water included in the shells, 
replacing the decomposed softer parts of gasteropods, etc., has in 
time affected the surrounding rock; (2) the castings of annelids 
have become dolomitized in preference to the surrounding rock; 
(3) the limestone immediately surrounding decomposing algae 
has been dolomitized, the magnesium salts liberated from the algae 
being sufficient to raise the percentage of Mg ions in the sea water 
so far that recrystallization could take place. 
The writer considers the third suggestion that which best 
explains the facts of the case. How far the effect is to be attrib- 
uted to fucoids, and how far to unicellular algae of the plankton, 
one cannot definitely ascertain; but fucoids probably played by 
far the main part in contributing the necessary magnesian salts. 
