STRUCTURAL RELATIONS OF PRE-CAMBRIAN AND PALAEOZOIC. 21 
ness. Well records in the St. Hyacinthe district show the Queens- 
ton to have a thickness of not less than 1,000 feet. The total 
thickness of beds lying above the Treiiton in the Montreal 
section is, therefore, about 3,300 feet. These beds superim- 
posed on the Trenton of the Mount Royal section would give 
a Palaeozoic section rising 3,800 A.T. If for the moment we 
neglect the existence of the Laurentian Plateau fault and project 
this 3,300 feet of sediments on the section constructed by Adams 
and Leroy 1 the top of the Palaeozoic section at Montreal would 
stand more than 3,000 feet above the average elevation of the 
Laurentian plateau lying 25 miles northwest of Montreal. 
Thus the structural feature which constitutes the subject of 
this paper may be ignored without invalidating the conclusion 
that the nearly horizontal sediments of the Montreal region 
must have reached far to the north of the present Archaean 
border. The recognition of a great subsidence fault at the 
southern border of the Archaean, however, makes the conception 
of a limited northern extent for the Palaeozoic still more improb- 
able. The total thickness of the Palaeozoic of the Montreal 
district, if we take the minimum figures of Adams and Leroy 
for the beds below the Utica-Lorraine, is about 5,025 feet. If 
the Laurentian scarp is the result of late Palaeozoic or Post- 
Palaeozoic faulting the sea floor of the early Palaeozoic was not 
interrupted by any inequality of grade near the present southern 
border of the Archaean. The old Pre-Cambrian land surface 
near Montreal must, therefore, have been depressed nearly 
a mile below sea-level in order to permit the accumulation of 
the 5,000 feet of Palaeozoic. We must postulate as an accessory 
to this subsidence at Montreal a regional depression of the areas 
to the north which permitted the Ordovician sea to invade 
much if not all of the Archaean area north of the Ottawa and 
St. Lawrence rivers. The elevation of the land near the divide 
between the Hudson Bay and the Ottawa-St. Lawrence drainage 
systems does not greatly exceed that of much of the southern 
part of the Laurentian plateau. East of Abitibi lake the highest 
^dem, sheet No. 874, 1905. 
