186 
THE GEOLOGIST. 
has to stand its cliance of being obliterated by beat, or waslicd away 
by water. 
' As all sedimentary strata are deposited from water, it follows that 
for every cubic yard deposited a cubic yard must be denuded from 
some other place ; and as the sedimentary rocks are much more com- 
mon at the surface, and generally softer than the igneous ones, the 
burden of supplying the sediment falls chiefly on them. We may 
therefore feel sm-e that during any one period nearly as many fossili- 
ferous strata are obliterated as are formed. In fact the power of 
denudation is so gi'eat, that Mr. Darwin and many other geologists 
think that only deposits formed during periods of subsidence are thick 
enough to resist its foice, so that many species, and even genera, that 
had but a limited range may have been swept away, and all record 
of their existence destroyed. 
This denudation added to the periods of repose will make the 
intervals between strata represent collectively far more time than the 
strata themselves, and we have many proofs that this is true in the 
numerous foreign strata that are intermediate in age to some of ours, 
in unconformability of stratification,* and in the abrupt change in 
the organic remains of consecutive formations. 
Three-fourths of the globe are covered with water, therefore three- 
fourths of the sti ata that remain are hidden from us ; and the other 
fourth has to be divided among all the formations that have as yet 
been recognised, for we can but examine the surface. Of the fourth 
that is accessible, not more than a fifth has been geologically explored ;t 
and that only where sections happen to exist. We must also re- 
member that large tracts of country, shown as Silurian, Devonian, &c., 
on om' maps, are covered so deeply with drift and alluvium that they 
never have been, and perhaps never ^vill be examined. 
For all these reasons the geological record must be very imperfect, 
and when we examine it we find such to be the case ; for we have no 
reason to suppose that the globe was less thickly inhabited in old 
times than now : on the contrary, when we find fossils at all they 
are generally in great abundance ; yet the number in any one forma- 
tion is almost as nothing compared to the number of living animals 
and plants. 
Mr. Darwin has justly observed " that in order to get a perfect 
gradation between two forms in the upper and lower parts of the 
same formation the deposit must have gone on accumulating for a 
very long period, in order to have given time for the slow process of 
variation, hence the deposit will generally have to be a very thick 
one ; and the species undergoing modification ^vill have had to 
live on the same area thi^oughout this time. But we have 
* The conformability of one stratum to another is no proof of its close se- 
quence ; for strata are sometimes conformable in one place, and unconformable 
m another. 
+ By explored I mean the age of its strata weU made out, not simply guessed 
