THE IRON-FORMATION ON BELCHER ISLANDS 437 
In the article previously cited on the origin of the Missouri 
cherts* it has been demonstrated that the presence of carbon dioxide 
has a very important effect in precipitating colloidal silica in the 
presence of calcium carbonate, causing it to be thrown down very 
quickly. It would therefore appear that in the action of carbon 
dioxide on colloidal silica and on the processes of the iron bacteria 
we may have a clue to the cause of the distinct banding in some of 
our pre-Cambrian iron-formations, provided further studies of these 
rocks tend to show evidence of the wide distribution of plant life 
during pre-Cambrian time. The fact that living algae will furnish 
oxygen to the waters around them and when they decay give off 
a certain amount of carbon dioxide may cause some seasonal varia- 
tion in the precipitation of the iron and silica, and thus give rise 
to the banding in some of these rocks. The distinctness of the 
banding may later be increased by metamorphism with recrystalliza- 
tion of the minerals and a certain amount of transfer of materials 
among the bands under the influence of chemical affinity. 
SUMMARY 
The Belcher Islands, which lie about seventy miles from the 
southeast coast of Hudson Bay, have recently been brought to the 
attention of geologists through the discovery on them of large 
areas of iron-formation. The iron-formation forms part of a 
thick series of sediments consisting of limestones, shales, quartzites, 
and graywackes, and this series is intruded by sills and overlain 
by flows of diabase and basalt, making up a group of rocks which 
in many respects strongly resemble part of the Animikie and 
Keweenawan formations of the Lake Superior region. The lime- 
stone of this group is, however, very unusual, since it consists of 
concretions varying from one inch to over fifteen inches in diameter 
and so strongly resembling some of the modern concretions formed 
by blue-green algae that there seems to be little doubt that they 
are of algal origin. They bear some resemblance to Cryptozoon 
proliferum, but differ from that fossil too much to be placed in the 
same genus. Their abundance indicates the presence of vast 
numbers of low plants in the Hudson Bay basin in pre-Cambrian 
1G. H. Cox, R. S. Dean, and V. H. Gottschalk, op. cit., pp. 9-10. 
