Rev. O. Fisher—Thickness of Marine Deposits. 209 
gone; and the author speculates among other causes upon the 
expansion and contraction of the underlying rocks, owing to changes 
of temperature arising from its being situated in a volcanic district, 
and he is thence led to speculate on the larger question of the 
depression and elevation of regions of the earth’s surface through 
the fall and rise of the isogeotherms, caused by denudation and 
deposition respectively. He concludes his paper (which he had 
kept by him for many years) with the following sentence: “ It may, 
however, be remarked that, whilst the principles on which it is 
founded are really existing causes, yet the sufficiency of the theory 
for explaining all the phenomena can only be admitted, when it shall 
have been shown that their power is fully adequate to produce all the 
observed effects.” 
In this passage Babbage shows his perception of the truth that, 
while certain known causes may undoubtedly operate in a certain 
direction, the question of “how much?” must be answered before 
it can be decided whether the phenomena can be attributed entirely 
to them. 
Without knowing what Babbage had been doing, Sir John Herschel 
communicated a similar theory about the rise and fall of isogeotherins 
in a letter from the Cape, dated February, 1886, and sent to Lyell’ 
His opinion appears to have been in most respects that which I have 
myself formulated, namely, that the Harth’s crust? rests in a state 
of approximate hydrostatic equilibrium® upon a molten substratum. 
The law of hydrostatic equilibrium is simply, that the weight of a 
mass floating in a liquid is equal to the weight of the volume of 
fluid which it displaces. If the crust of the earth was perfectly 
flexible this law would be applicable to every vertical prism into 
which it might be supposed divided; but since it is not perfectly 
flexible we must consider areas which are large in comparison of 
its thickness, and then, if material were to be denuded off one area, 
it would rise, because it was lightened, while the area to which the 
material was transferred would sink, because it was weighted. All 
the while the effective level of the liquid substratum would not be 
altered by the mere transfer of material. If the thickness of the 
crust is only twenty miles, or thereabouts, such an area as the delta 
of a large river would probably admit of the adjustment taking 
place: much more might we expect this to occur were the area 
comparable to that of a continent, or to the seaward extension of 
the detritus from one. 
This being premised, let us now endeavour to find answers to the 
two following questions of “how much ?”—(1) what thickness of 
deposit under certain conditions could be accumulated in a sea of a 
given depth? and (2) to what height above the sea-level could the 
deposit be subsequently elevated owing to the access of heat from 
beneath ? 
Let us then imagine a continent bordered by a not very deep 
1 Proce. Geol. Soc. vol. ii. pp. 548 and 596. 
2 Physies of the Earth’s Crust, chap. x. 2nd edition. 
8 American geologists have called this ‘‘ Isostacy.’’ 
