STRUCTURAL RELATIONS OF PRE-CAMBRIAN AND PALAEOZOIC. 
IS 
plateau border line before they received a cover of Palaeozoic 
sediments the strata on either side would have been of precisely 
the same lithologic type and no surface feature in any way com- 
parable with the present scarp could have resulted. Pre-Cam- 
brian faulting, therefore, fails as an explanatory hypothesis 
of the origin of the scarp. 
If, however, we assume that the transgressing Cambrian 
sea found no interrupting wall or line of cliffs where the southern 
margin of the plateau now stands, and that its sediments and 
those of succeeding Palaeozoic seas overspread the Archaean rocks 
of the present Laurentian plateau, then conditions were ideal 
for the production of an escarpment in the event of profound 
faulting followed by long continued denudation. Conspicuous 
physiographic evidence of such a fault would probably begin 
to appear only after erosion had removed the Palaeozoic beds on 
both sides down to the level of the uppermost Archaean. After 
denudation had reached this level its progress would be immeas- 
urably more rapid on the south or downthrow side. The 
major part of the beds to the south of the fault line being lime- 
stone, and to the north quartz and feldspar, their rate of degrada- 
tion would be roughly proportional to the resistance to erosion 
and solution offered by limestone on the one hand and granite 
on the other. The inevitable result of such differential erosion 
would be an escarpment of Archaean rocks, though this would 
be interrupted by irregularities in the original topography. Such 
irregularities are rather common along the southern front of 
the plateau just as they are in the more recently and less com- 
pletely uncovered Pre-Cambrian topography of the region about 
the head of the St. Lawrence. Like the latter they doubtless in 
many cases represent inequalities in the Pre-Cambrian land 
surface and some of them probably represent minor faults trans- 
verse to the main line of faulting. 
It will be seen from the foregoing considerations that, while 
Pre-Cambrian faulting would have been inadequate, normal 
faulting in the latter part of the Palaeozoic, or later, would have 
been entirely competent to produce this escarpment which the 
