782 A. P. COLEMAN 
as surface lava flows, and in part as glass fragments due to explosive 
eruptions. ‘The time required for all these manifestations of activity 
from the one eruptive hearth extended from the Lower Huronian to 
the Cambrian or a somewhat later period of the Paleozoic. 
Just why the nickel eruptive magma with its great masses of ore 
and its varied segregation products should occur near Sudbury and 
nowhere else in the vast Archaean region of northern Canada, there are 
no means of deciding; but prospectors familiar with the ore and its 
accompanying rock have looked for them far and wide, but in vain. 
Pyrrhotite has been found in considerable amounts in other places, 
but not in association with norite and with only a minute percentage 
of nickel. 
The Sudbury nickel-bearing sheet is unique as far as America is 
concerned, and far surpasses in magnitude and economic importance 
the small areas of nickeliferous norite of Scandinavia and other parts 
of Europe. Its great marginal ore deposits may be compared in size 
and origin with the segregations of titaniferous magnetite connected 
with large gabbro areas, and the percentage of iron which they contain 
would make them valuable ores of iron but for the presence of sulphur. 
