762 A. P. COLEMAN 
apt expression, the bowl of the spoon being filled with sedimentary 
rocks everywhere dipping inward. It rests upon the steeply upturned 
edges of Huronian schists as well as on Laurentian gneiss and some 
eruptive masses later than the rocks just mentioned. It is evident 
that the sheet separates two series of rocks having entirely different 
attitudes, and that originally the upper sediments rested uncomform- 
ably on the lower rocks. In Dr. Daly’s “Classification of Igneous 
Intrusive Bodies” it corresponds to an “‘interformational laccolith.”’ 
As will be seen from the accompanying map, the sheet has an 
outcrop of irregular width varying from five-sixths of a mile at the 
narrowest point of the northern range to four and one-fifth miles at the 
widest part of the main range. The average width is two miles and 
a half. As exposed at the various mines and prospects the basic 
edge dips inward, at angles from 20° to 64°; while the sedimentary 
rocks overlying the sheet have an average dip of about 30°. 
Accepting 30° as the correct average dip, the thickness of the 
sheet is a mile and a quarter. The syncline is 36.2 miles long by 
16.6 miles wide, with an average width of 13.6 miles; so that its 
solid contents must be about 600 cubic miles, if the concealed parts 
are as thick as the exposed edges. 
The upturned edges of the sheet have no doubt undergone great 
erosion, since the region has been exposed to erosion since Cambrian 
times. If we suppose that a width of three miles has been removed 
all around, the original mass must have been 1,000 cubic miles. In 
magnitude, then, the Sudbury laccolithic sheet far surpasses even 
the great sills described by Dr. Daly from British Columbia. 
As the term ‘sill’? seems to imply a flat-sided body rather than a 
curved one, and as sills regularly lie between two layers of sedimen- 
tary rock, I have preferred the non-committal term “sheet” for the 
Sudbury eruptive, which is synclinal and rests on a somewhat irregu- 
lar surface made up of a complex of schistose and eruptive rocks. 
CAUSE OF THE SYNCLINAL FORM 
One naturally asks why the eruptive sheet with the overlying 
sedimentary series should form a synclinal basin. As the surround- 
ing and underlying rocks show hardly any sympathy with this syn- 
clinal arrangement, we cannot assume that it is due to lateral mountain 
