MICHIGAN ACADEMY OF SCIENCE. 
133 
a third of a mile of continental rock. That would he a heavy load to 
he carried hy a crustal sheet which was already 3,000 to 5,000 feet or 
more above its normal level. If crustal yielding were so very tardy 
as that it is hard to understand why other sheets, all over the world, 
should yield at anything like the same altitude or at the same time. 
The pendulum investigations of isostasy* have shown that the devia- 
tions from normal altitude with respect to mass are more properly ex- 
pressed in hundreds of feet than in thousands. In general, isostatic 
balance is found to he nearly perfect, that is, within a very few hundred 
feet. 
Again, while formerly much reliance was placed upon the shrinkage 
of the cooling earth, that source of crustal motion is now known to 
have been greatly overrated. 
Ice-weighting can hardly he invoked to explain the supposed sinking 
(3,000 to 5,000 feet) on the coast of California because that was outside 
the region of general and heavy glaciation, hut mountain-making 
occurred near bv at about the same time that the sinking is thought 
That 
to have occurred and might perhaps be adduced in explanation, 
mountain-folding does produce depression is expressed in numerous 
foredeeps the world over. 
Admittedly one of the best-determined cases of river-drowning, is that 
of the Congo, 6,000 feet. In this instance neither ice-weighting nor 
mountain-weighting can have played any part, and through very long- 
geological time Africa has been subject to loss of mass through sub-aerial 
denudation, which ought to make it a rising continent rather than a 
sinking one. Africa itself presents absolutely no explanation of the sub- 
mergence of the Congo canon, especially in connection with the rest of 
the submerged margin, and unless we are able to reject the conclusions 
of isostasy, to which it is unlikely that many, at least of American geol- 
ogists, will agree, we shall find ourselves compelled to boldly accept 
a eustatic positive movement as the explanation and done with it. 
Again as to the depression of the glaciated lands by the weight of 
the ice; the density of the continental masses being not far from three, 
(water as unity), it would take about three miles of ice to depress the 
continental mass one mile. While there may possibly have been that 
much ice, or more, at some places, during the Glacial Epoch, it would 
seem to be an excessive figure for most of the glaciated area. From a 
half to one mile would probably be nearer the truth. 
The upward movement of the strand cannot be accounted for by de- 
formation of the lithosphere because since the oceans are thrice as 
extensive as the land, if all the lands on earth were depressed a mile 
and there were no submergence at all, no overflowing, so as to get the 
maximum amount of displacement, the ocean could only rise a third 
of a mile while all along the east coast of the Atlantic we have well 
over a mile to account for. 
Nor have we, under isostasv, anv true observed cause for such de- 
*Putnam, G. R., ‘‘Results of a Transcontinental Series of Gravity Measurements,” Phil. Soc. Wash. 
Bull., vol. xiii, 1895, pp. 31-60. 
Gilbert, G. K., “Notes on Gravity Determinations by Mr. Putnam.” Phil. Soc. Wash. Bull., vol. 
xiii, 1895, pp. 61-76. 
Hay-ford, John T., “The Figure of the Earth and Isostasy.” U. S. Coast and Geodetic Survey, 1909. 
Wiilis, Bailey, “Principles of Palaeogeography.” (section on permanence of ocean basins) Science, 
N. S. voi. xxxi, Feb. 18, 1910, pp. 243-246. 
Willis, Bailey, “What is Terra Firma? A Review of Current Research in Isostasy.” Ann. Rep. 
Smithson. Inst. 1910, pp. 391-406. 
