2 
MUSEUM BULLETIN NO. 22. 
this district the rocks are nearly all Pre-Cambrian. It is true, 
the Palaeozoic sediments to the south overlap the Pre-Cambrian 
among the islands in the North channel; but these younger 
strata do not occur in any of the localities under present considera- 
tion. Pleistocene lake-beds and glacial till also lie directly upon 
the Pre-Cambrian; but, beyond concealing the bed-rock, they 
have no relation to the problem in hand. Attention may, there- 
fore, be confined to the Pre-Cambrian. 
A great unconformity divides the Pre-Cambrian of the 
district into two main parts, Huronian and pre-Huronian. 
The pre-Huronian, in the various portions that have been 
mapped, consists principally of batholithic granite-gneiss. Meta- 
schists of igneous origin, older than the granite-gneiss and 
greatly metamorphosed by it, occur in much smaller amounts. 
Near Espanola there are also some highly metamorphosed 
quartzites and micaceous slates, older than the granite-gneiss. 
The age of these sediments relative to that of the igneous schists 
is not yet known. The pre-Huronian may, then, be sum- 
marily described as a. highly crystalline complex, consisting 
mainly of batholithic granite-gneiss and subordinately of 
vestigial patches of igneous schists and sediments intruded by 
the granite-gneiss. 
The Huronian sediments repose upon a deeply eroded sur- 
face of pre-Huronian crystallines. They are comparatively 
little metamorphosed deposits, almost flat-lying in places, closely 
folded and much faulted in others, but on the whole much more 
modern-looking than the pre-Huronian rocks. They consist 
dominantly of clastic materials — conglomerate, quartzite, and 
greywacke — with which are associated a few thin formations of 
siliceous limestone. The sequence of these formations is given 
below. An unconformity separates them into a lower, Bruce 
series and an upper, Cobalt series. This unconformity is a per- 
sistent one; but, because the two series have been deformed and 
anamorphosed to nearly the same extent, it is not conspicuous. 
The Huronian sediments are intruded in many parts of the 
district by sills and dykes of diabase of later Pre-Cambrian, 
probably Keweenawan, age. Near Thessalon, they are overlain 
by a basic volcanic flow. The present article is written to show 
that they are also intruded by granite of later Pre-Cambrian age. 
