56 ; General Notes. | January, 
region of aqueous action, they would be attacked by the water and 
their average specific gravity lowered. Now in case superficial ero- 
sion were to exceed internal erosion, the result would be a lowering 
of the continents ; but any lowering of the continents would reduce 
the rate of mechanical erosion much faster than it would the 
chemical, because very feeble springs and the mere capillary up- 
draught of saturated water, would remove the solid ingredients of 
the continents and place them in position to be drawn off to the 
sea by currents too feeble to bear much solid material in suspen- 
sibn. The specific gravity of the continents would, by this means, 
be continually lowered, and the oceanic areas as continuously 
loaded, and, for this reason, we might expect the continents and 
oceanic basins to persist. Again, even if we suppose the same 
degree of porosity to exist in the sedimentary beds under the 
ocean as exists in those of the continents and the materials of the 
two to have the same specific gravity, the same number of feet of 
sediment under the ocean would be heavier, volume for volume, _ 
than the land because, if for no other reason, the beds would 
be, in all probability, more fully saturated with water. Now Pro- 
fessor Ferrel has shown that the attraction of continental plateaus 
must be neglected in reducing both pendulum and barometric ob- 
servations to sea level, and therefore they do not represent so 
much material added between a given station and the earth’s cen- 
ter; that is, these earth masses, although possessing longer radii, 
are no heavier than equal sections in the ocean areas. 
Assuming that the continents and ocean beds, with their super- 
incumbent. water, are essentially in equilibrium, and taking the 
average depth of the oceans as 15,000 feet and the average height 
of continents, above sea level, as 1000 feet, we could obtain a 
tolerably accurate estimate of the average specific gravity of the 
continents if we knew the average density of the rocks below the 
sea bottom, knowing, as we do, the specific gravity of 15,000 feet 
of superimposed matter. The specific gravity of the earth 400 
miles below the surface is estimated at 4.0478 (U. S. Coast and 
Geodetic Survey, 1879), and our heaviést known rocks scarcely 
run above 3. From these considerations, and from what we know 
of the specific gravity of sedimentary rocks, we should not expect 
the sedimentary beds of the sea bottom to have a specific gravity 
much above 2.5. Assuming an average of 2.5 for the first 5000 
feet below sea bottom and of 2.95 for the next 10,000 feet, then 
the average specific gravity of the continental mass required to 
exactly balance this would be 1.851, assuming, of course, that 
a surface of uniform density under both oceans and continents is 
reached at a depth of 30,000 feet below the sea level. Now con- 
sidering the specific gravity to increase below 15,000 feet below 
sea bottom at the rate of .05 for every 10,000 feet downward, it 
would then be necessary to go to a depth of about thirteen miles 
below sea level to obtain an average density sufficiently large to 
a 2 š ive rece 
Pe pS BEE GE ES aR Pn RN RTE E 
