THE SUDBURY LACCOLITAIC SHEET 767 
but there are examples near each end of the basin where squeezing and 
shearing have gone so far as to produce schists. As these are often 
greenish in color and quite unlike granite or gneiss, they proved very 
puzzling in the field, but the microscope shows that they consist mainly 
of plumy micropegmatite with a little chlorite, and an analysis gives 
a composition similar to the granitic portions of the acid edge, as 
may be seen from column nine of the table of analyses on a later page. 
Judging from its mineralogical composition, most of the acid 
portion of the eruptive sheet is grano-diorite, but parts of it have so 
little plagioclase that they might be called granite, while other parts 
with comparatively little potash feldspar might be called quartz-diorite. 
RELATION OF THE BASIC EDGE TO THE ROCKS BELOW 
As may be seen from the map, the basic’ edge of the laccolithic 
sheet is, in places, quite irregular, with inward projections of the 
country rock and bays directed outward. From some of the funnel- 
shaped bays narrow bands of norite reach out into the country rock, 
sometimes for several miles, but usually with gaps in their continuity. 
For these projections I have used the term “offset” rather than 
“dike,”’ since the latter implies a continuous sheet of eruptive rock 
with somewhat uniform width. One or two of these offsets may be 
described to bring out the relations of the nickel eruptive to the rocks 
on which it rests. 
Where the country rock extends like a promontory into the basic 
edge there are no ore bodies, but where the opposite occurs, the norite 
pushing baylike into the country rock, as at Creighton, large ore 
bodies are found. When such a bay opens outward into an offset, 
as at Copper Cliff, the ore forms only small deposits along the sides of 
the funnel, but larger bodies where interruptions break the offset. 
In the Copper Cliff offset, for example, we find first a wide bay, with 
three small masses of ore along its edges; then a narrow band extends 
southeast for a third of a mile, ending in No. 2 mine, a body of ore 
230 feet in length along the offset, and half as wide. This chimney- 
like deposit has been worked to a depth of 400 feet, and probably goes 
much deeper. 
To the south of this the offset is cut off for a third of a mile, but 
reappears at the famous Copper Cliff mine, with an ore body 75 to 200 
