170 
STRUCTURAL GEOLOGY 
is probably in excess of 25 miles. The existence of such extensive 
overthrust is rather unexpected along a mountain front that has 
■ always been believed to be one limb of a great crustal plait. 
The chief significance of the overthrust at Crow’s Nest is not 
that it is anything novel or rare. It does give, however, an idea 
of what to expect in other parts of the Cordillera, even where 
such effect is little suspected. Heretofore Rocky Mountain fault¬ 
ing has been entirely limed on the normal-slip basis. That some 
of these faults should prove to be thrusts is most illuminating. 
That others will doubtless turn out to belong to the same category 
is now to be expected. Even some of the great faults of the 
Grand Canyon region and the High Plateaus now need careful 
examination anew in order to determine whether on this basis 
some of the long inexplicable features recorded do not find more 
satisfactory explanation. The same is true of the desert ranges 
of the Great Basin and of the Mexican tableland. 
In so many respects recently have our notions concerning the 
nature of western tectonics undergone fundamental change that 
in regard to the dislocative features we may have soon to add 
another revolution of ideas. K^yes. 
Biplanation of Barth’s Straticulate Crust. Always contraposed 
to the facial wrinkling of our planet because of orographic trouble 
is a smoothing out of the selfsame inequalities of relief by agen¬ 
cies which we designate erosive. Whether the planing off process 
be accomplished by the waves of the sea, which we distinguish 
^as marine denudation, by regional corrasion of streams, which is 
known as base-leveling, or by provincial wind-scour, which is 
termed deflation, a flattened surface is the final effect and the 
ultimate relief expression is a plane lying close to sea-level. 
Peneplanation is the leveling off of the upper, or outer, surface 
of the earth’s crust. It is the smoothing out process which alone 
is usually considered. There is, however, another planation of 
the earth’s crust of which little or no notice ever has been taken. 
It is that of the lower surface of the straticulate layer. Epicene 
powers are not involved. Yet the truncation of the folded strata 
and stratified structures is probably as real and as extensive as 
the other with which we are more familiar. Passage from the 
zone of rock-fracture to that of rock-flowage, as characterized by 
schistose structures, is the occular demonstration of the existence 
of the process. 
