256 Rev. O. Fisher—Thickness of Marine Deposits. 
sea, and that the deposition of sediment has not yet commenced. 
The isogeotherms under both areas would then be sensibly stationary, 
and the flow of heat steady under both areas. Next suppose denu- 
dation to commence over the continental area, and that sediment is” 
deposited on the sea bottom. The land will begin to rise, and the 
sea bottom to sink. Whether this goes on equably, or by jerks 
accompanied by earthquakes, does not concern us now. By the law 
of hydrostatic equilibrium, the thickness of the deposit possible will 
depend upon the relative densities of the liquid substratum, and of 
the crust, which is pressed down into it, because a limit will be imposed 
by the obvious circumstance that the deposit must cease to accumu- 
late, at least as a marine deposit, as soon as it reaches the top of the 
water. After this has happened, the detrital matter will be pushed 
forward beyond the new coast line thus formed, so that on the large 
scale the entire deposit will have a character of false bedding. 
The simple calculations on which the following estimates are 
based are given in the appended note. - 
We are obliged to make a guess as to the densities of the crust, 
and of the liquid substratum on which it floats. I see none more 
probable than those which I have assumed in my Physics of the 
Earth’s Crust, viz., 2°68 (a fair average for granite) fur the crust, 
and 2°96 (a fair average for basalt) for the substratum. We ought 
also to know the density of the deposit, which may be put with 
considerable probability at 2°5, after consolidation. Since the accu- 
mulation of the deposit will depress the crust into the substratum, 
we have to consider whether it is likely that the original crust will 
maintain its thickness unaltered, or whether some of it will be melted 
off. It is obvious that on the average the substratum, where it is 
in contact with the underside of the crust, cannot be able to supply 
heat to the crust more rapidly than it is carried off by conduction to 
the surface, otherwise the average crust would be constantly growing 
thinner, instead of thicker as it must do. Still, if the bottom of the 
crust was depressed into the substratum, it might be argued that it 
would reach hotter layers, where it would be melted. But since the 
heat in a liquid capable of convection cannot become rapidly hotter 
from the surface downwards, it does not seem probable that much 
melting would occur from that cause. But on the other hand, the 
increased thickness arising from the addition of the deposit would 
retard the flow of heat, and since there is reason to think that the sub- 
stratum is somewhat above the melting temperature,’ some thinning 
of the crust below might arise from this cause. The most probable — 
hypothesis appears to be that, after a sufficient time has elapsed, the | 
action of the substratum will have reduced the new crust, which 
will consist of the deposit and what remains of the original crust, to 
the average thickness, that is to the original thickness of the crust at 
the place in question. But we have in the appended note considered 
1 See the Author’s Physics of the Earth’s Crust, ch. vi. Prof. Bartoli found the 
temperature of lava issuing from an underground gallery at Etna, in 1893, to be 
1060° C., and after flowing a mile it was 870° C., and still liquid. This shows that 
it was above the melting temperature at its first appearing. 
