DHE SUDBURY EACCOLIDEIC SHEET 
A. P. COLEMAN 
University of Toronto. 
TABLE OF CONTENTS 
Introduction. 
Shape and Size of the Sheet. 
Cause of the Synclinal Form. 
Petrography of the Nickel Eruptive. 
Relation of the Basic Edge to the Rocks Below. 
The Upper Contact of the Laccolithic Sheet. 
Chemical Composition of the Sheet. 
Causes of Differentiation. 
Earlier and Later Derivatives of the Magma. 
Basic Norite. 
Granite. 
The Ramsay Lake Gabbro Band. 
Other Eruptives. 
Time Relations and Conclusion. 
INTRODUCTION 
The Sudbury mining region has for nearly twenty years attracted 
the attention of mining engineers and geologists because of its great 
deposits of nickeliferous sulphides associated with a special eruptive 
rock. Recent field-work carried out by the writer and his assistants 
for the Bureau of Mines of Ontario has demonstrated that the region 
is even more interesting from the geological side than had been sup- 
posed; since the ore bodies form part of the edge of a great eruptive 
sheet having a length of 36 miles, a breadth of 16 miles, and a thick- 
ness of a mile and a quarter. While cooling this molten sheet under- 
went magmatic segregation in which gravitation played a large part, 
so that the heaviest ingredients, the ores, sank to the lowest points, 
merging upward into norite, the next heavier rock, which passes 
upward into granite, the lightest rock of the sheet. 
Details of the geology of the region may be found in a final report 
by the writer recently distributed by the Bureau of Mines;? but as 
t Bureau of Mines (Ontario, 1905), Part III. 
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