THE IRON-FORMATION ON BELCHER ISLANDS 431 
triangular to nearly spherical. A few are long and narrow and some 
are curved (Fig. 17). One measured 1.70 millimeters in diameter. 
Some granules consist of opal, in others the opal is changing to 
chert by loss of water, and the chert is in turn changing to granules 
of quartz by crystallization. Some granules consist chiefly of 
magnetite, some of hematite, and in others a smattering of both 
occurs throughout the silica. In a number of granules consisting 
chiefly of iron oxide the grains of silica are arranged in groups, so 
that they produce under the microscope a cell-like structure strongly 
Fic. 18.—A. Photomicrograph showing the hematite and greenalite granules 
with grains of silica distributed in a cell-like arrangement. The dark concretions are 
apple green to red and brown. The lighter areas are silica (X20). From the cherty 
iron-formation. 
B. From the same specimen as A. 
resembling what would result if a fragment of a bryozoan were 
replaced by iron and silica. Its regularity in so many granules is 
suggestive. 
The most interesting specimen was taken from the siliceous 
shales in the upper part of the iron-formation, where they underlie 
the basalt on Kasegalick Lake. In the hand specimen it is a dense, 
grayish-black to light-gray, cherty rock containing dark, cherty 
grains. It weathers to a dark, brownish mass. Under the micro- 
scope it is seen to be made up almost entirely of granules of various 
shapes, colors, and sizes (Fig. 18, A and B). In shape they are 
