382 
GEOLOGY: B. WILLIS 
which registers the effects at the surface more quickly in one region or 
more slowly in another, as the case may be. 
Do the mechanics of deformation of the lithosphere, as expressed in the 
effects of compression, conform to the structures which should follow from the 
postulated discoidal structure? — Under general tangential stress a spherical 
shell composed of discs and interdiscs should shorten by riding up of 
the margins of the discs upon the interdiscs. If the disc were competent 
to sustain itself by partial bridging, its whole mass would rise; if, as 
must generally if not always have been the case in the lithosphere, the 
disc be too wide and too weak to support itself, the margins only would 
be raised, the mass bending or fracturing as conditions demanded. The 
marginal zones should exhibit over-thrusts from the basin side and 
incidental phenomena of folding with minor over- and under thrusting. 
The facts of mountain structure appear to conform to these require- 
ments. The folded and over thrust structures of the Appalachians and 
the faulted and upthrust structures of the Pacific ranges of California 
appear to find an intelligible explanation on the basis of the discoidal 
theory. 
Do the trend lines of mountain chains conform in plan to the postulated 
discoidal structure? — To answer this question in detail will require an 
analysis of the earth's surface into discs and interdiscs, but the trend 
lines of the mountain chains of Europe, Asia, and the two Americas, 
as developed by Suess, do conform in plan to the obvious depressions 
within and around those continents. It must be recognized, however, 
that a long narrow depression filled with sediment may be so com- 
pressed and displaced as to become part of the continental interdisc. 
This is a well known geological occurrence, the transformation of a 
geosyncline into a mountain range, but it cannot in every such case 
be assumed that the geosyncline corresponded to a dense underbody. 
Other mechanical conditions may produce subsidence. With this 
reservation, however, I believe that the directrices of mountain chains 
can be shown to wind around discoidal masses. 
Does the discoidal theory suggest any solution for the temporary inter- 
continental land connections which are demonstrated by biologic evidence? — 
The present isthmian connection between North and South America 
may be regarded as a typical case of intercontinental bond established 
by marginal uplift and extrusion of igneous rocks between the Carib- 
bean and eastern Pacific discs Similar connections may readily be 
traced between South America and Africa by two different routes. 
A connection between North America and Europe, via Greenland, 
Iceland, and England, such as has often been suggested, finds a rat'onai 
