14 
MUSEUM BULLETIN NO. 18, 
and in many cases at right angles to it, the improbability, if 
not impossibility, of this hypothesis becomes manifest. If 
synclinal structure were a competent explanation a consequent 
general southerly inclination of the beds north of the areas of 
maximum thickness would manifest itself in the numerous 
outcrops along the Ottawa river. In all of the outcrops known 
to the writers, however, the beds are horizontal or nearly so, 
except in the immediate vicinity of faults. In the section pas- 
sing through Aylmer the general dip is even to the north. Near 
faults the dips, though often heavy for short distances, die 
out in a few yards or rods. Examination of the accompanying 
sections will show that most, if not all, of the declination of the 
beds is due to normal faults. The assumption of a synclinal 
trough structure could in no degree explain the scarp-like face 
which marks the southern border of the Laurentian plateau. 
We may next consider the hypothesis of the development 
in pre-Palaeozoic times of a Laurentian escarpment bordering 
the Palaeozoic lowland. Such a feature cannot be assumed to 
be the possible product of folding or down-warping in Pre- 
Cambrian times, for, as noted above, the general strike of the 
crystalline rocks is everywhere transverse to the line of the scarp. 
Any trough-like structure which might have been developed 
through folding or the differential erosion of folded strata would 
have a northeast and southwest trend instead of the general 
east-west course of the northern border of the Palaeozoic rocks. 
If pre-Palaeozoic faulting is assumed to have produced the es- 
carpment as the northern rim of a graben type of depression, as 
suggested by one author 1 for the Montreal district, it will be 
profitable to consider the conditions under w T hich scarps may be 
developed in connexion with faults. In the southern Alle- 
ghanies, faults with throws of from 5,000 to 8,000 feet cut the 
Palaeozoics and are not betrayed by the slightest physiographic 
evidence. The great faults in southwest Virginia may be cited 
as an illustration of the general principle that faults result in 
scarps only when beds of unequal hardness are exposed to erosion. 
If faulting had occurred in the Archaean rocks along the present 
1 A. W. G. Wilson, Jour. Geol., Vol. XI, 1903, p. 620. 
