8 
MUSEUM BULLETIN NO. 18. 
Cambrian time enveloped an Archaean land surface of highly 
irregular character but of moderate relief. It is probable that 
the higher hills of that topography seldom rose more than 300 
or 400 feet above their adjacent valleys. The section (Figure 
6), from the Ottawa river to Lake Erie across the Ontario 
peninsula, indicates clearly the entire absence of any discordance 
in the general profile of the pre-Palseozoic surface in that region, 
and there is no evidence of the existence elsewhere in the Pre- 
Cambrian topography of any marked or abrupt changes in the 
general level of the land such as that marked by the southern 
escarpment of the Laurentian plateau. The irregularly etched 
surface of the old Pre-Cambrian land, therefore, affords no ex- 
planation of so unique a topographic feature; and this discrep- 
ancy is all the more apparent when we realize that this escarp- 
ment abruptly truncates the regularly oriented structural fea- 
tures of the region (see page 12). 
STRUCTURE OF THE PALAEOZOIC PLAIN. 
The great expanse of Palaeozoic lowland which borders the 
Ottawa and St. Lawrence valleys for 200 miles is separated from 
the Palaeozoic area of the Ontario peninsula by a belt of Pre-Cam- 
brian rocks, very narrow at the head of the St. Lawrence but 
widening rapidly to the northwest. This area of Archaean rocks 
emerges like an island from the low-lying Palaeozoic cover to the 
east and to the southwest, a relationship which is clearly shown 
by the cross-section, Figure 6. That this shield-like uplift took 
place subsequent to the time of Ordovician sedimentation is 
indicated by outliers of the Ordovician such as that at Clear 
lake. 1 This outlier rests upon the Pre-Cambrian at an elevation 
about 830 feet A.T., while on either margin of the Archaean shield 
sediments of the same age are found at elevations of 100 or 200 A.T., 
and these pass below sea-level toward the southwest, east, and 
northeast. On the east, owing chiefly to faulting, the elevation 
of these beds varies widely in different places ; but to the south- 
west they decline, along the line of section (Figure 6), at a uni- 
form rate of 12| feet per mile until, at Toronto, the base of the 
1 Ann, Rept. Geoi. Surv., Canada, Vol. XIV, 1904, p. 7J. 
