THE ITRON-FORMATION ON BELCHER ISLANDS 433 
be seen that they are very similar and that these Upper Cambrian 
odlites are associated with abundant cryptozoon fossils. Further, 
if the various types of odlitic iron ores from the Clinton and Cam- 
brian of America, from the Silurian of Europe, and from the Jurassic 
of France and England be examined, similar bodies will be found. 
In fact, so far as the writer’s experience goes with slides from the 
iron-bearing rocks of the formations later than the pre-Cambrian, 
such concretionary bodies are only found in those rocks which con- 
tain other evidence of organic action. 
Although the writer recognizes that low forms of life, such as 
algae and iron bacteria, are not essential to the formation of odlites* 
and related concretions in all cases, it seems probable that they 
generally serve as the agents which produce the chemical changes 
causing precipitation of the calcium carbonate and the iron. This 
action seems to be due chiefly to the removal of the carbon dioxide 
from the acid carbonates by the algae, and the oxidation of the 
iron by the iron bacteria, thus in both instances producing insoluble 
compounds. The occurrence of vast deposits of odlites and related 
concretions in certain geological formations, in some instances on 
different continents during the same period, is also suggestive of the 
influence of certain organisms which reached a high stage of develop- 
ment at that particular time and caused the precipitation of the 
iron or other salts. It may also be due to the particular conditions 
of erosion, which permitted a large amount of any particular kind 
of salt to be carried to the sea during a certain geological period. 
In his monumental works on the origin of the pre-Cambrian 
iron ores in various parts of the continent Dr. Leith has held firmly 
to the belief that the bulk of the iron has been supplied directly 
to the sea as magmatic waters accompanying the great eruptions 
of basic igneous rock. He advocates this theory very strongly for 
the iron-formations on the east coast of Hudson Bay.? Although 
the writer readily recognizes the possibility of supplies of iron salts 
from this source, he cannot see that they have played a rdle at 
all comparable to the supplies carried into the sea, or into inland 
« An additional note on ‘‘The Oolitic and Pisolitic Barite from the Saratoga Oil 
Field, Texas,” Science, N.S., XLVI (October 5, 1917), 342. 
2121s Leithops cit pa ean. 
