STRUCTURAL RELATIONS OF PRE-CAMBRIAN AND PALAEOZOIC. 
13 
tact of the older and younger series of rocks is marked by ter- 
races of marine clays which in some degree conceal the abrupt 
transition from a plain to a plateau scarp. At some points 
offset faults to the north or south appear to have interrupted 
the continuity of the scarp. A combination of these two fea- 
tures in the district immediately north and northeast of Ottawa 
obscures the abrupt and generally observable contact of the two 
topographic types. Northwest of the lower Gatineau valley 
the Laurentian escarpment continues uninterruptedly for nearly 
20 miles, rising 800 to 1,000 feet above the lowland at its base. 
The irregular character of the Pre- Cambrian topography has 
already been pointed out (pages 4-8). Denudation of the 
Palaeozoic rocks has uncovered this ancient hilly topography 
in many places along the northern border of the present Palae- 
ozoic plain, so that the Palaeozoic plain is sometimes separated 
from the base of the plateau by a border of irregular Archaean 
hills. These hills of granite and schist occasionally have an 
elevation of 200 or 300 hundred feet, although generally less, 
above the adjacent Palaeozoic rocks. Where hills of this type 
are present the south-facing escarpment of the Laurentian 
plateau may be obscured, but its position is generally easily 
recognized by a pronounced change in the relief. 
INTERPRETATION OF THE DATA. 
In the preceding pages we have shown that the Laurentian 
plateau, for approximately 300 miles, rises abruptly several 
hundred feet above the Palaeozoic plain and that a zone in which 
faulting is the dominant structural feature borders this escarp- 
ment on the south. We will now consider the significance 
of this topographic discordance and of the associated structural 
features. 
The possibility of synclinal structure affecting both the 
Palaeozoic and Archaean series at once suggests itself, to one 
unfamiliar with the field relations, as an explanation of the 
Laurentian escarpment. When it is pointed out, however, 
that the older series has been highly folded and that the general 
trend of this folding is across the course of the escarpment 
