304 The American Geologist. November. 1894 
the most part there is no evidence to show that they occupy 
the sites of preglacial river valleys. With few exceptions 
their courses were determined by the conditions existing 
during the period of the disappearance of the ice-sheet. To 
understand those conditions clearly, a brief glance at the pre- 
glacial aspect of the Carboniferous area will prove of value. 
The close of the Devonian found this area isolated from the 
main ocean, having been entirely surrounded by deposits of 
that age. It, however, retained one or more outlets through 
which its surplus waters were discharged. During the nu- 
merous oscillations to which the land was subjected during 
Carboniferous time this area was alternately an open sea in 
which heavy sandstones were deposited and an extensive 
swamp sufficiently stable to support a luxuriant flora. In 
either case there was always a large amount of surplus water, 
and hence the existence of the outlet was probably continu- 
ous. Now, when this inland sea was finally filled by the con- 
tinued formation of off shore deposits the central part was 
presumably the newest and hence the lowest, giving a slightly 
concave form to the surface ; the concavity being broken 
through at the points occupied by outlets. It seems probable 
that these outlets were two, one to the east discharging its 
waters through the St. Lawrence valley, the other flowing to 
the west for some distance and then south to the ocean. 
Starting in this condition the area was subjected for some 
millions of years, during Mesozoic and Cenozoic time, to un- 
interrupted erosive influences. The result was a transforma- 
tion of the circular concave surface to a trough-like configu- 
ration ; the main drainage channels occupying the line of 
least elevation across it, and the northern and southern por- 
tions sloping to the center. At this stage of the erosive 
process the country was overwhelmed by the glacial invasion. 
It is already obvious to the reader that the present configura- 
tion is not particularly different from that just outlined. The 
central depression is still a marked feature, and the two lat- 
eral portions incline gently towards it. The glacial forces 
modified it, but could not obliterate it. Its origin therefore 
dates from the close of the Devonian age. We are now pre- 
pared to consider intelligently the origin of our river systems. 
Glacialists are generally agreed that the direction of ice 
