20 
MUSEUM BULLETIN NO. 8. 
They do not often dip more than 45 degrees, while the pre-Huron- 
ian sediments beneath stand on edge or nearly so and the green 
schists have a similarly steep schistosity. Also the a-rkose 
found at the base of the Huronian in many places was derived 
from the pre-Huronian directly beneath, while the conglomerate 
which forms the basal member elsewhere contains pebbles and 
boulders of a great variety of pre-Huronian formations. It 
is evident, therefore, that the unconformity between the Huron- 
ian and pre-Huronian is a positive and strongly marked one. 
When Logan and Murray worked in the country north of 
Lake Huron sixty years ago they were impressed by the unlike- 
ness of what are here called the Huronian and pre-Huronian 
rocks. The former are little metamorphosed, and not greatly 
folded sediments, while the sediments and schists of the latter 
are closely folded, with more or less prominent secondary cleav- 
age, are greatly metamorphosed, and are intruded by great 
batholithic masses of granite-gneiss. The significance of this 
difference between the Huronian and pre-Huronian and of the 
unconformity between them was expressed a year ago: “The 
Cobalt series rests upon a crystalline rock surface as maturely 
eroded and peneplain-like as the present surface of the region. 
The full chronological importance of the break is perhaps most 
apparent near Sudbury, where Cobalt conglomerate rests upon 
the upturned edges of the Sudbury series and upon the gneiss 
that intrudes the Sudbury series. In the interval between the 
deposition of the Sudbury series and the Cobalt series, the for- 
mer was affected by orogenic movements, the granite batholiths 
were intruded, and the mountains so formed were reduced to a 
peneplain. Some conception of the time required for these 
changes may be gained from a consideration of the Rocky 
Mountain region, where seemingly analogous processes have been 
in progress since Jurassic time at least and are still far from com- 
plete.”^ The Cobalt series referred to in this extract is the same 
as the upper Huronian series of the present paper and the struc- 
tural relations described for Sudbury are the same as those 
found north of Lake Huron. 
^ "A classification of the Pre-Cambrian formations in the region east of 
Lake Superior”, page 406. 
