r72 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
of 76 miles, and shows, moreover, that that depth is almost certainly 
not less than 62 miles, nor greater than 87 miles. 
Pendulum experiments also show that there is a deficiency of 
gravitating matter beneath mountain ranges and table lands, and an 
excess near the seashore. This has been especially observable in 
India, where at elevated points near the Himalayas the value of the 
force of gravity has been found to be the same as it would have been 
if there were no intervening mountain mass between the point of 
observation and sea level. On the other hand, in the neighborhood 
of the Indian Ocean an excess of attracting matter was revealed. 
Facts like these first led Archdeacon Pratt to suspect the existence 
of isostasy. 
We see, however, various agencies at work on the earth’s surface 
that must tend to disturb this isostatic condition. Mountains are 
being worn down by the action of water and their materials trans- 
ported to lower elevations, and there deposited. If, then, this state 
of isostasy is maintained, it must be effected by a counter flow of 
material in the opposite direction somewhere below the surface. 
Geology furnishes evidence that this takes place by the fact that in 
spite of rapid denudation mountain regions are often observed to 
maintain their elevation, as if they were raised from below as fast 
as they are torn down from above. Also sedimentary rocks of great 
thickness, such as the Paleozoic formations of the Appalachian 
region, contain shallow-water fossils throughout, showing that the 
sea bottom must have sunk as fast as sediment was piled upon it. 
Similar examples occur in many other localities. 
On the theory of isostasy the interior portion of our globe, within 
the layer of compensation is composed of material of the same density 
at equal distances from the center—or rather, the layers of equal 
density are concentric spheroids. The theory that the crust of the 
earth is only a few miles in thickness, and rests upon an intensely 
heated molten interior, is no longer tenable. It is now known that 
the earth as a whole possesses a high degree of effective rigidity, as 
great as if it were composed throughout of steel. It is no doubt true 
that the interior of the earth is in an intensely heated condition and 
that it appears to possess some of the properties of a fluid; at the same 
time it behaves in many respects as a solid. 
A heated and therefore cooling body like the earth must shrink; 
and thus its solid outer crust would be continually under the necessity 
of adapting itself to a contracting interior. This would give rise to 
enormous tangential stresses in the crust to which it must eventually 
yield. That this has taken place in the past is evidenced by the plica- 
tions and dislocations shown in the rock strata that compose the 
crust; and to the fact that it is still taking place are probably due the 
earthquake shocks that are of almost daily occurrence in some part 
of the world. It is practically certain that no earthquake center has 
