THE SUDBURY LACCOLITHIC SHEET 769 
lower edge; but, on the other hand, has exerted a far greater meta- 
morphic action. No long dikes or apophyses are known to project 
from the acid edge into the overlying rocks, though some short ones 
have been observed. 
The rocks above the laccolithic sheet are the Trout Lake conglom- 
erate with an estimated thickness of 450 feet, the Onaping tuff, 
3,800 feet thick, the Onwatin slate, 3,700 feet thick, and the Chelms- 
ford sandstone, 350 feet thick, giving a total of 8,300 feet of sediments. 
The lowest of these formations, the Trout Lake conglomerate, 
which was in direct contact with the upper surface of the molten 
sheet, has been strongly metamorphosed; and about half of the next 
higher one, the Onaping tuff, has been greatly indurated and silicified, 
though its fragmental character is still distinct. ‘The overlying sedi- 
ments are comparatively soft and unchanged, having a very modern 
look; while the Trout Lake conglomerate has been so metamor- 
phosed in many places as to suggest a Laurentian gneiss, and was in 
fact so mapped in earlier days. 
The three upper formations are fairly uniform in thickness, but 
the Trout Lake conglomerate varies from more than 600 feet to only 
20 feet, partly perhaps because of original differences in thickness 
natural to so coarse-textured a rock as a bowlder conglomerate, but 
partly also, it is believed, because unequal amounts have been 
absorbed by the underlying eruptive. 
The lower part of the conglomerate has been so completely recon- 
structed that little can be said as to its original composition except 
that it inclosed large and small bowlders of granite, and that the 
matrix had a composition which could be transformed into gneiss. 
In less altered parts well-rounded pebbles and bowlders of granite, 
of quartzite, and of green schist or greenstone are found in a grayish 
crystalline matrix. Occasionally white quartzite is interbedded with 
the conglomerate. 
The exact boundary of the micropegmatite against the conglom- 
erate is very hard to trace. Often in our field-work when crossing 
the edge of the eruptive toward the sediments, we would overrun the 
limit, which could be recognized only by the coarser-textured and 
redder patches representing granite bowlders. 
Thin sections made of hand specimens taken across the boundary 
