18 
MUSEUM BULLETIN NO, 18. 
border of the Laurentian plateau. The senior author has examined 
several areas where stream valleys cross from the Archaean 
to the Palaeozoic, in order to discover, if possible, a single case 
where the Palaeozoic extends up a valley into the Archaean 
area; but no such case has been found and the contact appears 
to be everywhere a series of approximately straight or gently 
curving lines. 
Another class of facts which may be considered in this 
connexion is the lithologic character of the limestones at or near 
their northern border. The Trenton limestone at Ottawa is 
a nearly pure limestone entirely free from sand, pebbles, or other 
detrital material which the granite hills rising a thousand feet 
above its border must have supplied to it if these hills formed 
the northern boundary of the Trenton sea. The freedom 
from detrital material of this and other limestones of the Ottawa 
section is one of many facts which indicate that the present 
topographic relations between the Palaeozoic and Archaean 
series are the result of faulting subsequent to their formation. 
The position of the fault which is responsible for this topographic 
inequality is shown in Figure 5. It may be conveniently des- 
ignated as the Laurentian Plateau fault, since it marks the south- 
ern margin of this upland throughout the area in which it im- 
pinges on the Palaeozoic rocks. At Quebec, it approaches very 
near to the line of the St. Lawrence and Lake Champlain fault, 
the two being almost parallel and lying only a few miles apart 
northeast of the city. The Laurentian Plateau fault is supposed 
to pass into the river below the Island of Orleans. North of 
the city of Quebec the southern border of the plateau is broken 
up by valleys into a series of more or less detached hills. Round- 
top, which is representative of these, has an elevation of 1,600 
feet (barometer). The Palaeozoic plain has here a width of only 
4 or 5 miles; Archaean hills 1,200 to 1,600 feet high limit the 
plain on the northwest, and a tableland comprised chiefly of the 
highly deformed Sillery beds borders it on the southeast. This 
plain rapidly widens in ascending the St. Lawrence valley above 
Quebec. Between Quebec and St. Anne river the line of the 
fault is believed to lie north of the margin of the Palaeozoic, 
as shown on the map, at the foot of the line of high Archaean 
