56 
DUTTON. 
or with gigantic faults measuring their immense uplifts, 
plainly declare to us that they have been slowly pushed up- 
.wards as fast as they were degraded by secular erosion. 
It seems little doubtful that these subsidences of accumu¬ 
lated deposits and these progressive upward movements of 
eroded mountain platforms are, in the main, results of grav¬ 
itation restoring the isostasy which has been disturbed by 
denudation on the one hand and by sedimentation on the 
other. The magnitudes of the masses which thus show the 
isostatic tendency are in some cases no greater than a single 
mountain platform, less than 100 miles in length, from 20 
to 40 miles wide and from 2500 to 3500 feet mean altitude 
above the surrounding lowlands. From this we may directly 
infer that in those regions the effective rigidity of the earth 
is insufficient to uphold a mass so great as one of those plat¬ 
forms if that mass constituted a real deformation of isostasy; 
and if an equal mass were to be suddenly removed the 
earth would flow upward from below to fill the hiatus; 
hence we must look to considerably smaller masses to find 
a defect of isostasy. It is extremely probable that small or 
narrow ridges are not isostatic with respect to the country 
roundabout them. Some volcanic mountains may be ex¬ 
pected to be non-isostatic, especially isolated volcanic piles. 
Thus the geologic changes which have taken place may 
be regarded as experiments conducted by Nature herself on 
a vast scale, and from her experiments we may by suitable 
working hypotheses draw provisional conclusions, both as to 
the degree in which the earth approximates to isostasy and 
also as to the mean effective rigidity of large portions of the 
subterranean mass. The approach to isostasy is thereby in¬ 
ferred to be very near, while the mean rigidity of the sub¬ 
terranean masses is also inferred to be far less than that of 
ordinary surface rocks, and even approaching more nearly 
the rigidity of lead than to that of copper. Pure physics 
alohe would not have enabled us to reach such a conclusion, 
for the equations employ constants of unknown value. But 
geologic inquiry may, and I believe does, furnish us with 
