Marca 15, 1901.] 
however, that the excavation of valleys or 
gorges like these by rivers, when the slope 
and water supply are favorable, occurs with 
such rapidity relative to the wider effects 
of denudation, as to be almost negligible in 
any general view of the physical changes 
of an extensive region or in the accounting 
of geological time. 
There is as yet some difficulty in connect- 
ing the later physical changes particularly 
referred to above with those which have 
recently come under observation far to the 
north in the Klondike region. It is prob- 
able, however, that the auriferous ‘ quartz 
drift’ of that region, implying long subaérial 
decay and stability of level, may be attrib- 
uted to the early Pliocene; while the river 
gravels found in the newer and deeper-cut 
valleys may be assigned to the later Plio- 
cene time of greater elevation. During the 
Pliocene, and probably until its close, the 
mammoth, one or two species of bison, the 
moose and other large mammals roamed 
northward to the Arctic sea. Then came 
the Glacial period, with renewed great 
changes in levels and climate and its own 
peculiar records and history, which in many 
respects are more difficult of interpretation 
than those of more remote periods, because 
the whole time occupied by them has been 
relatively so brief. I have elsewhere en- 
deavored to follow this history in detail, 
and do not propose on this occasion to deal 
with this latest chapter of the physical 
history of the Rocky Mountain region of 
Canada. 
In conclusion, what appear to be the 
most striking points evidenced by the geo- 
logical record of this northern part of the 
Cordillera may perhaps be specified as fol- 
lows: 
1. The great thickness of strata ac- 
cumulated both to the east and west of an 
Archean axis. In the Laramide geosyn- 
cline the strata no doubt actually attained 
the volume stated. In the western and 
SCIENCE. 
407 
wider syncline it is not so certain that all 
the formations in their full thickness were 
ever actually superposed at any one place 
or time (for reasons already alluded to), 
but the volume was probably not less than 
in the Laramide region. 
2. The great proportion of volcanic ma- 
terials accumulated in the western geo- 
syncline and the recurrence of vulcanism 
throughout the geological time-scale in this 
region, resulting in the production of mass- 
ive voleanic formations in the Cambrian, 
Carboniferous, Triassic, Cretaceous and 
Miocene. 
3. The recurrence of folding and disturb- 
ance parallel to the border of the Pacific 
basin and the concurrent great changes 
in elevation of the land relatively to the 
sea, both continued down to quite recent 
geological times, the latter even into the 
Pleistocene. 
4. The tremendous energy of denuda- 
tion, in part due to the events last referred 
to, but also dependent upon the position of 
the region on the eastern border of a great 
ocean, where, in northern latitudes, an ex- 
cessive rainfall must have occurred at all 
periods on the seaward mountain ranges. 
No comparable denuding forces were prob- 
ably ever operative on the east side of the 
continent in similar latitudes since the 
definition of the ocean basins of the Pacific 
and Atlantic. G. M. Dawson. 
GEOLOGICAL SURVEY OF CANADA. 
STEREOSCOPIC STUDY OF THE MOON. 
In looking at a terrestrial landscape we 
see that certain features are distant and 
others near. We also recognize the exten- 
sion of objects in three dimensions, so that 
a tree, for example, is not a mere silhouette, 
but is perceived in its proper rotundity. 
The data for these automatic and instan- 
taneous judgments as to distance and form 
are somewhat complex. The distance of 
