90 
DYNAMICAL GEOLOGY 
by the situation of the main volcanic activities at the sharp solid 
angles, or the points where each set of three faces intersect. 
Viewed, then, in their larger or telluric relations the conti¬ 
nents are probably best regarded not as broad basins with up¬ 
turned rims but as somewhat irregular, interrupted meridionally 
disposed ridges. These ribs appear to be the directly traceable in 
their genesis to release of cumulative tension that depends upon 
the secular retardation of the earth’s rotation. 
Keyes. 
Geological Directrix of Isostasy. Seemingly insuperable diffi¬ 
culties which are encountered since geodists took up mathematic¬ 
ally the solution of isostatic compensation in the earth’s crust now 
appear to be not nearly so irreconcilable geologically as not so 
very long ago was quite generally supposed. The differences 
prove to be mainly one of viewpoint. 
As the prospects of the outdoor geologist and of the indoor 
mathematician become hopelessly confused it is discovered that 
there are actually under consideration two entirely distinct and 
essentially unrelated things. -The one resqlves itself into simple 
hydrostatics, which is, of course,' wholly independent of any geo¬ 
logical data which may be adduced; while the other passes into 
impossible tectonics. Recognizing the fact that the isostasy of 
Hayforth and Bowie is not the' isostasy of Dutton and Gilbert, 
Becker and Davis, among others, ascribe the isostatic incongrui¬ 
ties to tangential pressure and telluric crushing effects, which are 
clearly not isostatic properties at all. 
Geologically prime interest in the isostatic theory concerns not 
so much its verity or falsity as it does the minimum crustal arc 
over which it is appreciably effective in mountain construction. 
Such compensation is neither so far-reaching as mathematicians 
proclaim nor so perfect and delicate as geologists sometimes argue. 
When from recent gravity measurements Gilbert shows that some 
large mountain tracts are at least partially self-sustaining latest 
observations in western America seem amply to testify. 
It is not so surprising that these recent determinations should 
appear to demonstrate conclusively that isostatic compensation 
does not obtain in the case of single mountain ridges, as the iso¬ 
lated desert ranges of the Great Basin. But when it appears to 
