72 0. E. SCHIOTZ. . [NORW. POL. EXP. 
assume a somewhat more rapid decrease than the above equation gives. In 
any case, I assume that it follows from this equation that the difference 
between the densities in the firm crust beneath the continents and beneath 
the oceans can be only slight, so that the earth’s crust must have a com- 
paratively considerable thickness, if equation (I) is to be satisfied. 
It appears to me that with regard to the density nearest the surface at 
the bottom of the ocean, there is no reason for supposing that it is per- 
ceptibly different from that at the surface of the continents. The oceans in 
all probability have not always been so deep as they are now; the depth has 
increased little by little, as the earth’s outer shell has shrunk more and more 
together with the cooling down and contraction of the inner nucleus. If we 
look back in time, we must therefore suppose the oceans shallower and shal- 
lower the further back we go, taking for granted that on the whole they have 
always occupied the same places on the earth’s surface. When the firm 
earth’s crust was formed, there is no reason therefore to suppose it otherwise 
where the continents happened to he than in the other places where the 
oceans subsequently came to be. It is true that the surface of the continents, 
after the latter had risen out of the sea, was exposed to continual denudation; 
but as the eroded masses were once more deposited upon areas belonging to the 
continents, this denudation has principally brought about a re-adjustment of 
the masses at the surface of the continents, and presumably cannot, to any 
perceptible extent, have produced any alteration in the average density at the 
surface, if, as previously mentioned, we imagine the continents to have been 
reduced to the level of the sea by the employment of the accumulated masses 
to compensate for deficiency of mass below. By thus sinking deeper and 
deeper during the shrinkage, those parts of the firm earth’s crust upon which 
the ocean would rest were brought into contact with denser masses than those 
parts of which the continents were formed. On this account, I imagine, the 
density beneath the oceans came to increase more rapidly with the depth, 
than beneath the continents. 
If this is the case, it seems to me reasonable to suppose that those parts 
of the earth’s crust containing the boundaries of the continents, and which 
we have hitherto not considered, do not differ in any essential degree in their 
constitution from the other parts, but that in their case also we may suppose 
that on an average there is the same quantity of matter above every unit 
