CORRELATION OF THE EARLY SILURIAN ROCKS IN 
THE HUDSON BAY REGION? 
T. E. SAVAGE 
University of Illinois, Urbana, Illinois 
The oldest rocks of Silurian age known in the Hudson Bay 
region are present in the banks of Nelson River, about forty-five 
miles above tide-water. The best exposure is about four miles 
below the outcrop of Richmond strata at the lower Limestone 
Rapids of the river, where a vertical ledge outcrops to a height of 
28 feet above low water. The rocks are nearly horizontal or gently 
undulating, and consist of yellowish-brown, rather fine-grained 
dolomite, in layers 4 to 1o inches thick. Masses of this dolomite 
form a pavement along the banks of the river at intervals for 
several miles below the main exposure, indicating that the river 
is actively cutting into these strata in places east of their actual 
outcrop. 
The fossils in this dolomite appear to be restricted to a narrow 
zone in the lower part of the bed. The most abundant are molds 
and casts, mostly of the ventral valve, of shells of the species 
described by Whiteaves as Conchidium decussatum from the basal 
Silurian strata at the Grand Rapids of Saskatchewan River. 
These shells are in places so crowded together as to make up the 
greater part of the rock layers, just as they occur at the Grand 
Rapids outcrop, where they are also restricted to a narrow zone. 
The shells of this species found in the Nelson River region show 
a wide variation in the ratio of their length and width, in the degree 
of convexity or galeation of the ventral valve, and in the develop- 
ment of the mesial fold on the ventral valve. Some of the partially 
exfoliated shells even show a distinct mesial sinus extending from 
the beak over the umbonal region of the ventral valve, which 
becomes obsolete or is transformed into a mesial fold in the middle 
«The strata discussed in this paper probably fall within the later half of the 
Oswegan series of the New York Classification. 
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