THE SUDBURY LACCOLITHAIC SHEET 
building thrusts, but must look for some other cause. 
probable supposition is that the source of the magma was imme- 
diately beneath the longer axis of the area, 
which is now basin-shaped, but was then 
flat and undisturbed. 
The original molten mass was prob- 
ably much thicker, but also much nar- 
rower, than the present laccolithic sheet. 
When it ascended and spread out widely 
between the upper sediments and the 
less regular rocks beneath, there was a 
collapse, since the schists and older 
eruptives below had lost their central 
support. This shows itself very plainly 
in the shattered and faulted character of 
the rocks underlying the sheet. The 
collapse gave rise to the roughly synclinal 
basin occupied by the eruptive, while the 
overlying sedimentary rocks settling into 
the still plastic magma beneath formed 
a more regular syncline. 
The process of collapse and adjust- 
ment was probably a very slow one, 
beginning with the earlier upward 
movements of the magma, and ending 
only when the whole mass had cooled so 
far as to lose its plasticity. 
PETROGRAPHY OF THE NICKEL ERUPTIVE 
The petrography of this great sheet 
has been touched on by numerous writers, 
particularly since Baron von Foullon 
proved that the fresh rock associated 
with the nickel ore contains hypersthene 
‘qp'dy7am hoa +9) 
Wt |\292224/ 2710 
‘an 907 A 
P72~-mfroascy 
aqrzponip 
Murray Mine 
+ 
@ 
ae 
Chelmsford 8- 
Onwatin Slat 
—— 
OT 
Section Across Nickel District 
Seale q Miles 
763 
The most 
3 74 
a 
t 
row Norvthurest fa Southeast 
instead of hornblende, and is therefore norite and not diorite, as it 
was named at first. 
Details of the petrography may be found in 
