406 
only slightly lower than at present, and 
that the movements in subsidence and ele- 
vation between this and the interior region 
must have been differential in character 
and very unequal in amount. 
As already noted, the earliest Tertiary 
sediments of the Interior plateau of the 
Cordillera are referred to the Oligocene. 
Probably some further subsidence at that 
time interrupted the long preceding time 
of waste. This period of deposition was 
in turn closed by renewed disturbance of 
an orogenic kind, comparatively slight in 
amount and local, chiefly affecting certain 
lines in a northwest and southeast direc- 
tion. Next came renewed denudation or 
‘planation,’ and this continued until the 
enormous volcanic extravasations of the 
Miocene began. 
It is not proposed in this place to reca- 
pitulate in detail the physical conditions of 
the Tertiary period, for it has already been 
necessary to refer to these in connection 
with the description of the beds them- 
selves, which, because they have not been 
materially changed since their deposition, 
really tell their own tale. 
It need only be said that, after the Oligo- 
cene lake deposits had been formed, dis- 
turbed and denuded, new series of lakes 
were from time to time produced at differ- 
ent stages during the Miocene, their beds 
now generally appearing as intercalations 
in volcanic deposits of great mass. Both 
the coast and the interior region appear to 
have been subject to these conditions, 
while the Laramide range stood high, with 
the inland plain of the continent sloping 
eastward from its base. 
Following the close of, or at least a great 
reduction in, voleanic activity in the early 
Pliocene, the interior zone of the Cordillera 
again assumed a condition of stability for a 
considerable time, during which wide and 
‘mature’ stream valleys wereformed. The 
elevation of the interior plateau region of 
SCIENCE. 
[N. S. Vou. XIII. No. 324. 
British Columbia must then have been about 
2,000 feet less than it is at present.* Far- 
ther north, the yellow Pliocene gravels of 
Horsefly river and other places are attrib- 
uted to this period, and the southern as- 
pect of their contained fossil plants is such 
as to indicate that, in the given latitude, 
the height of that part of the interior can 
not have been much above the sea level. 
In the later Pliocence a very marked re- 
elevation of the Cordilleran region evidently 
occurred, leading to the renewed activity of 
river erosion, the cutting. out of deep val- 
leys and canyons, and the shaping of the 
surface to a form much like that held by it 
at the present day. This elevation in all 
probability affected the coast as well as the 
interior, and it would appear that the rivers 
for a time extended their courses to the 
edge of the continental plateau. 
The excavation of the remarkable fiords 
of British Columbia and the sonthern part 
of Alaska must, I think, be chiefly attrib- 
uted to the later portion of the Pliocene, 
although it is quite possible that the cut- 
ting out of the valleys may have been begun 
soon after the Laramide upheaval. The 
antiquity of these valleys is evidenced by 
the fact that several comparatively small 
rivers still flow completely across the Coast 
ranges in their deep troughs. The fiords 
are now essentially the submerged lower 
parts of these and other drainage valleys of 
the old land, not very materially affected 
by the later glacial action, important as 
this has undoubtedly been from other points 
of view. The valleys of the fiord-like lakes 
that occur along the flanks of the Archean 
axis of the interior may probably also be 
referred to river erosion in the later Plio- 
cene, but if so this mountain region must 
have been affected by a relatively greater 
uplift at that time, followed later by a 
subsidence of its central part. It appears, 
* Trans. Royal Soc. Can., Vol. VIII., Sec. IV., p. 
18. 
