STRUCTURAL RELATIONS OF PRE-CAMBRIAN AND PALAEOZOIC. 
23 
widely, if not completely, over the Laurentian upland southeast 
and east of Hudson bay. The present boundary between this 
Laurentian upland and the Palaeozoic lowland is believed to be 
the locus of a line of post-Ordovician normal faulting for which 
the term Laurentian Plateau fault is proposed. The existence 
of such a fault is supported by the following considerations: 
(a) the extreme regularity in the direction of the escarpment, 
(b) the abrupt truncation of the structural features of the Pre- 
Cambrian area by the escarpment, (c) the presence of outliers 
of the Ordovician upon the Pre-Cambrian, (d) the fact that the 
escarpment is located along the northern border of a zone in 
which subsidence or normal faulting is peculiarly characteristic, 
(e) the general horizontality of the Palaeozoic beds in the area 
south of the escarpment, (f) the usual absence of physiographic 
evidence of faults unless they result in the differential erosion 
of beds of unequal resistance, (g) the discordance between the 
physiographic features along the escarpment and along other 
nearlying borders of the Archaean where normal erosion has even 
yielded an escarpment of the Palaeozoic, (h) the absence of Palae- 
ozoic penetration into breaks in the escarpment, and (i) the entire 
absence of clastic material from the limestone immediately 
adjacent to the escarpment. 
