THE SUDBURY LACCOLITHIC SHEET 781 
rocks; followed by the collapse of the older rocks because of the 
removal of the molten mass from beneath, the result of this being 
the synclinal arrangement of the nickel eruptive and the overlying 
sediments. Some coarse-grained granite or granitoid gneiss was 
erupted at about the same time or a little before. Not long after 
the sheet was spread out and solidified, fine-grained granite pushed up 
through it or beside it; perhaps the last product of the original magma. 
The nickel-bearing sheet and the granites just mentioned were post- 
Huronian in age, perhaps Cambrian or even Ordovician. When all 
had cooled and hardened, including even the most fluid part of the 
magma, the ore in the hollows beneath the sheet, fissures cut across 
all the rocks and were filled with olivine diabase. Last of all a small 
supply of acid magma penetrated fissures crossing the diabase dikes. 
The date of this latest of the granites cannot be determined, since 
there are no fossiliferous rocks in the region. 
From the outline just given it will be seen how long and complex 
a history there is behind the Sudbury nickel ranges and their associated 
rocks, including sedimentary rocks of four ages, a great sheet of vol- 
canic ashes, norite or gabbro of three ages, granite of at least three 
ages, and dikes of diabase; the events beginning with the earliest 
known period of the earth’s history and ending in lower Paleozoic 
times. 
If the inferences suggested regarding the relationships of the erup- 
tive rocks are correct, the same magmatic source has provided medium 
gabbro, very basic norite, rather acid volcanic ash, coarse-grained 
granite, somewhat acid norite merging upward into pegmatite and 
downward into nickel ore, and acid fine-grained granite. Whether 
the much later dikes of diabase and granite should be placed in the 
succession is doubtful. 
The operations included the formation of coarse-grained plutonic 
rocks of a stocklike kind, flows of lava showing pillow and amygda- 
loidal structures, the flinging-out of vitrophyre tuffs mingled with 
some ordinary sediments, followed by the injection of the vast nickel- 
bearing sheet and its slow differentiation by gravity and the stoping 
and absorption of overlying rocks; so that the products of the original 
magma cooled in part very slowly at great depths, in part somewhat 
more rapidly as a sheet buried under 8,000 feet of sediments, in part 
