[ELLs] GEOLOGY OF THE OTTAWA PALÆOZOIC BASIN 109 
posures are rarely continuous over any large area, and the formations 
are effected by dislocations, often of considerable extent, many of which 
are, owing to the drift, difficult to locate. 
From the facts stated in the geology of Canada, 1863, as well as 
from other information obtained since that date at a number of points 
in the Ottawa-St. Lawrence basin, fairly correct estimates of the thick- 
nesses of the several formations found in the Ottawa district may how- 
ever be given. 
Of these presumably the most widely developed at the surface are 
the Potsdam sandstone and the Calciferous limestone. The former 
rests upon the denuded surface of the crystalline rocks at many points, 
and the lowest member is frequently an arkose or a conglomerate, made 
up of the debris of the limestone, gneiss and quartzite of the older series. 
Sometimes however, the lowest members of the Paleozoic series are 
absent, either from non-deposition or through the agency of faults; and 
sometimes the Chazy or even the Trenton is the lowest member of the 
fossiliferous rocks. This latter phase is well seen at the falls of the 
Montmorenci river below Quebec city. 
In a former paper on the “Paleozoic outliers of the Ottawa basin,” 
printed in the Transactions of this Society in 1894, some estimate of 
the thickness of several members of the Paleozoic formations was given. 
The information at hand at that date has since been extended, and pre- 
sumably more satisfactory details have now been obtained. In some 
cases, however, exact figures can not yet be stated, but a brief resumé of 
the knowledge at our disposal may be found useful as a basis for further 
observation, 
The thickness of the Potsdam sandstone is to some extent conjec- 
tural. There are no complete exposures of the strata of this formation 
available for measurement throughout the entire basin, but from a 
number of outcrops along the St. Lawrence and the Ottawa rivers it 
has been estimated at from 300 to 700 feet. Thus at Hemmingford, 
near the International boundary between the province of Quebec and 
the state of New York, a thickness of 540 feet was measured by Sir 
William Logan, but this did not apparently reach the transition beds 
into the Calciferous formation. Along the St. Lawrence, below Brock- 
ville, a section of the upper part of the formation measured seventy-five 
to eighty feet, passing into the dolomites of the overlying formation. 
Near Charleston village, on Charleston lake, a section of the lower 
portion, from the Archean upward, measured seventy-one feet without 
reaching the Calciferous. 
Near the village of Lachute, on the north side of the Ottawa, an 
exposure shows about forty feet of the sandstone, but this includes 
