GEOLOGY 187 
so that at the time of their deposition the Cambro-Silurian sea 
occupied a great basin open to the Arctic ocean and extending 
southward into the middle of the continent. 
This was the time of maximum encroachment of the northern 
ocean, after which the land gradually rose, and the sea slowly 
receded. Owing to the great lapse of time and the eroding of 
the thick ice-cap in the more southern regions, it is exceedingly 
difficult to now trace the boundaries of the narrowing sea dur- 
ing silurian and Devonian times. Cambro-Silurian limestones 
containing fossils which refer them to the Galena-Trenton, 
are widespread over the northern islands and in a wide margin 
along the western and southern shores of Hudson bay. 
outliers of these rocks occur at the head of Frobisher bay in 
the southwest part of baffin island and on Akpatok island in 
ungava bay. As before stated, similar limestones are found in 
the lake valleys of Manitoba, and it is quite possible that these 
limestones were once continuous with those of Hudson bay, the 
present break having been caused by the erosion of the glacier. 
The upward continuation of these limestones containing 
Silurian fossils occupies a corresponding but slightly circum- 
scribed area. These Silurian limestones form the characteristic 
abrupt cliffs of the islands on both sides of Lancaster sound, 
and continuing southward occupy the larger parts of Southamp- 
ton, Coats and Mansfield islands in Hudson bay, as well as the 
low lands of the western part of Baffin island. They are not 
well marked, and are probably considerably thinner on the 
western side of Hudson bay, but are found in the Winnipeg 
basin. 
the Devonian gradually emerges from the Silurian in the 
cliffs of the islands to the north of Lancaster sound, and forms 
the lower parts of the cliffs of the southern side of Ellesmere. 
Devonian fossils are not found in the limestones of the islands 
of Hudson bay, and only occur in a narrow belt on the low lands 
