FIELD RELATIONS AND SECTIONS. 231 
and Shebandowan sheets—the work of Lawson, McInnis, and Smith. 
As the scale is small and the lake and river systems are complicated, it 
seemed wise to omit much of the topography so as not to confuse the 
geological boundaries. The portion mapped includes on its western side 
one of the most characteristic batholitic areas, though tracts to the north 
and west would serve equally well as illustrations if the geological sheets 
covering them had been published. 
Only a few of the granite bosses are mapped, many being too small to 
be conveniently shown. 
SECTIONS THROUGH PRE-CAMBRIAN MOouNTAINS 
HKvidently this well glaciated Archean peneplain presents a most in- 
structive section through the base of an ancient group of mountains. The 
sharp synclines of Couchiching and Keewatin nipped in between areas of 
gneiss and granite are merely remnants preserved because of their pro- 
tected position. Often unfinished curves may be seen running out into 
the Laurentian, erosion having eaten below the bottom of the part that 
has vanished. Imagining these roughly circular or oval synclinal folds 
complete, we should see convex surfaces forming domes of diameters 
varying from 1 mile to 50 miles. What their height was in their prime 
one can hardly guess ; but from their diameter and the steep dip of their 
synelinal portions one may suppose that the largest of these pre-Cam- 
brian mountains were comparable with the greatest mountains of the 
present day. 
The process by which they were built seems to have differed from that 
of later times in the earth’s history, since they were not constructed of 
a series of parallel anticlinal folds, but were pushed up more like the 
laccolitic mountains described by Gilbert and Whitman Cross from the 
western United States, though with the difference that the up-pushing 
mass of molten rock was broad based. 
While the larger areas of gneiss and granite are evidently batholites, 
some of the smaller granite bosses may be stumps of old volcanoes, 
though they could not have furnished the acid pyroclastics of the Kee- 
watin through which they rise. Lawson looks on the Shoal Lake an- 
orthosite boss as the stump of a voleano furnishing basic pyroclastics.* 
GEOLOGICAL History oF THE REGION 
Reviewing the facts presented by the rocks of western Ontario, we find 
an oldest series, the Couchiching, consisting of sands and clayey sands 
* Geol. Survey of Canada, 1887, p. 57, F. 
XXXV—Butt. Guot. Soc. Am., Vou. 9, 1897. 
