Rev. O. Fisher—Thickness of Marine Deposits. 207 
these two cases—firstly that in which none of the crust is melted 
off, and secondly that in which the thickness of the deposit and 
old crust‘together is reduced to that of the original crust. The 
results are that, in the case of none being melted off, the thickness 
of the deposit which could accumulate, so as just to reach the surface 
of the sea, would be 4:26 times the depth of the sea. And in the 
second case, where the thickness of the deposit and old crust together 
is reduced by melting off to the original thickness of the crust, the 
depth of the deposit to fill up the sea would be 10-9 times the depth 
of the sea. These estimates depend upon the densities assumed ; 
but they do not require a knowledge of the thickness of the original 
crust. 
We have next to consider the rise of the isogeotherms produced 
by the covering, or “blanketing”’ as it has been called, of the crust 
by the new deposit, and to estimate the resulting expansion of both. 
This is the important question started by Babbage, but, so far as I 
am aware, no estimate has been attempted to be made on the 
hypothesis of a liquid substratum. In passing it may be remarked 
that the ascertained fact, that the crust has sunk while being loaded, 
is a very strong argument for the presence of a liquid substratum.’ 
It is clear that a temperature, perhaps rather above that of fusion, 
would be maintained beneath the bottom of the crust, and that heat 
would flow upwards out of the old crust into the fresh deposit ; 
and also out of the substratum into the old crust. Meanwhile the 
temperature at the top of the deposit would always be that of the 
sea-bottom, which we may consider to be zero; and the temperature 
of the bottom of the crust may be taken to be the melting tempera- 
ture. We shall be sufficiently near the truth if we consider the 
crust, when under normal conditions, to show a regular increase of 
temperature with the depth, say one degree F’. for 51 feet of descent ; 
and if the deposition of sediment was to cease, an equable rate of 
increase would after a long time be re-established in the area under 
consideration, though perhaps with a lower gradient than the normal 
one on account of a possible increase of thickness of the crust; and 
the longer the time, the more exactly would this equable rate be 
attained. Suppose then, for the sake of argument, that the flow of 
heat was to be held in abeyance until the sea was filled up. The 
surface of the water being reached, it is clear that no further 
marine deposit could take place. As soon as this is accomplished, 
suppose the heat to be free to flow again. Then after the lapse 
of a long interval the flow will again become steady, and the 
gradient become an equable one, but until this is accomplished 
more heat will enter from below than escapes at the surface; so 
1 Astronomers admit this. Professor Newcomb says ‘‘ We have next to consider 
the effect of viscosity of the earth. Those geologists who have given special atten- 
tion to the subject regard it as well established that the earth yields under the 
weight of deposits as if it were a thin crust floating upon a liquid interior and there- 
fore must be a viscous solid if a solid at all.’’—Monthly Notices of the Roy. Ast. 
Soc., March, 1892. 
Prof. Harkness of the U.S.A. Navy writes of the plausibility of the theory of 
hydrostatic equilibrium of the crust in his great work on the Solar Parallax, etc. 
DECADE III.—vVoL. X.—NO. VI. 17 
