72 O. E. SCHI0TZ. [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 lie 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 



