2 
MUSEUM BULLETIN NO. IS. 
The geologic significance of a physiographic feature having 
so great an areal extent as this is of great interest. It has been 
generally regarded as part of the southern shore-line of the 
Pre-Cambrian nucleus of the American continent. Another 
interpretation of the meaning of this feature was first suggested 
to the authors by a cross-section of the rocks from Ottawa to 
Lake Erie, prepared by the junior author. The senior author 
has undertaken by field studies along the northern border of 
the Palaeozoics to secure evidence bearing upon the several 
hypotheses which have been considered in attempting to explain 
the relations which subsist between the Palaeozoic lowland and 
the Pre-Cambrian upland. The topographic contrasts shown 
by these two physiographic types are indicated in Plate I. 
Three hypotheses appear to merit consideration in any 
attempt to ascertain the relations which the Palaeozoic and Pre- 
Cambrian rocks bear to each other along the southern face of 
the Laurentian plateau. These are: (1) synclinal structure of 
the area adjacent to the southern border of the Laurentian 
plateau; (2) pre-Palaeozoic development of the Laurentian 
escarpment, and (3) subsidence or normal faulting of post- 
Ordovician age. The evidence for and against these several 
hypotheses can be more clearly presented and understood after 
a brief consideration of the stratigraphic relations of the Pre- 
Cambrian and Palaeozoic formations and of the known struc- 
tural features of the latter. 
GENERAL STRATIGRAPHIC RELATIONS. 
The Palaeozoic rocks south of the Ottawa river rest upon a 
series of Pre-Cambrian rocks from which they are separated 
by a great unconformity. The older series includes a wide 
range of types of crystalline intrusive rocks intimately associated 
with ancient sediments which have been subjected to intense 
metamorphism. The character of these Pre- Cambrian rocks 
is well shown both in the region immediately north of the Ottawa 
river and in the crystalline area to the southwest of the Ottawa- 
St. Lawrence Palaeozoic area. In the latter region Adams and 
Barlow report an enormous thickness for the Archaean rocks. 
