1886.] Geology and Paleontology. 55 
tion of density near the earth’s surface, remove all materials of 
low density from the continents and place them over the sea bot- 
tom, while the elevation of the denuded region would bring den- 
ser materials to the surface, thus tending to restore equilibrium 
with the two surfaces more nearly on the same level, unless 
there is some agent operating to reduce the average specific grav- 
ity of the continents. 
If the earth does increase in density toward the center, this may 
be due : first, to a difference of chemical composition ; or second, 
to increasing pressure; or third, to these two conditions in combi- 
nation. With either the first or third conditions existing, and 
continued denudation with no counter agent, a leveling up would 
inevitably result. With the second condition existing, unloading 
in one place and loading in another of equal area, would permit of 
expansion in the continental mass and cause a compression of 
strata under the oceans, and might maintain the differences of level 
already established ; but this view being very improbable, it re- 
mains to search for some cause which may reduce the specific 
gravity of the continents, and an adequate one, it seems to me, may 
be found in internal chemical and mechanical erosion. 
Taking Mr. T. M. Read’s estimate of chemical erosion (Am. 
Jour. Sci., April, 1885), at 100 tons per square mile annually on 
the average the world over, as a fair estimate of the work done 
by the waters which come to the surface before emptying into the 
ocean, it is plain that a vast work must be done in reducing the 
average specific gravity of the continents, unless it is maintained 
that the small cavities produced are closed by compression as fast 
as formed. This certainly is not the case in the superficial strata, 
nor can it be the case in the deeper strata where the cavities pro- 
duced by solution remain filled with water. 
Data are altogether too meager to allow of a quantitative treat- 
ment of the question. We do not know, for example, what propor- 
tion of the matter carried in solution to the sea by rivers annually 
is obtained through purely superficial action. Neither do we 
know what proportion of the water falling upon the continents 
enters the ocean below ocean level. It is reasonable to suppose 
that this amount is not small, and that the water entering the sea 
below ocean level carries a higher per cent of solids than the 
average river water. Now that our Government scientific work is 
being consolidated, it would seem eminently fitting that these fun- 
damental questions should occupy the joint attention of the U. S. 
logical Servey and the Signal Service, and they are possibly 
already under consideration. 
This internal erosion, by excavating small cavities in the body 
of the continents, would lighten them without in the same degree 
lowering their surfaces, and existing differences of level would be 
longer, if not permanently, maintained, because in case the denser 
strata were to be thrust up into the heart of the continents, into the 
