STRUCTURAL GEOLOGY 
171 
In an area undergoing extensive sedimentation and concompe- 
tent down-sinking the bottom of the column of the older sediments 
may reach depths that are beyond the limits of fracture. The 
extralimital portion flows off and, undergoing recrystallization of 
constituent components, reappears as schists from which all evi¬ 
dences of sedimentary origin may have disappeared. Although 
perhaps- not so sharply defined, the level at which the change takes 
place must be essentially a plane. So that the folded straticulate 
crust is really bevelled above and below. It may be that many of 
the low-angled thrust-planes which we always regard as expres¬ 
sions of compressive movement are really phenomena of under- 
crustal planation. 
The expression of the phenomenon in diagram is represented 
in the annexed cut (fig. 11). The figure also indicates the char¬ 
acter of the mobile datum plane from which geology estimates the 
intensity of isostasy. KeyEs. 
Flexures in Canadian Front Ranges of Rockies. Notably close 
folding in the northern section of the Rocky Mountains presents 
strong tectonic contrast ta the open flexing characterizing the 
southern part of the Cordillera. The extremely appressed features 
displayed on the west side of the mountain axis, where the pre- 
Cambrian strata measure 30,000 feet in thickness, suggests possible 
fan-structures which are usually associated only with the Alps, 
made classic by Heim and other of the Swiss geologists. 
But it seems wholly unnecessary to travel to far-off Switzerland 
in order to view typical Alpine structures in mountain-building. 
Jasper Park, in northwest Alberta, appears to afford as fine dis¬ 
plays of this description as any of those of which we know abroad. 
