296 
IMPERFECTION OF THE 
Chap. IX. 
order to enable tlie same species to live on the same 
space, the supply of sediment must nearly have counter¬ 
balanced the amount of subsidence. But this same 
movement of subsidence will often tend to sink the 
area whence the sediment is derived, and thus diminish 
the supply whilst the downward movement continues. 
In fact, this nearly exact balancing between the supply 
of sediment and the amount of subsidence is probably 
a rare contingency; for it has been observed by more 
than one palaeontologist, that very thick deposits are 
usually barren of organic remains, except near their 
upper or lower limits. 
It would seem that each separate formation, like the 
whole pile of formations in any country, has generally 
been intermittent in its accumulation. When we see, 
as is so often the case, a formation composed of beds 
of different mineralogical composition, we may reason¬ 
ably suspect that the process of deposition has been 
much interrupted, as a change in the currents of the 
sea and a supply of sediment of a different nature will 
generally have been due to geographical changes re¬ 
quiring much time. Nor will the closest inspection of 
a formation give any idea of the time which its depo¬ 
sition has consumed. Many instances could be given of 
beds only a few feet in thickness, representing forma¬ 
tions, elsewhere thousands of feet in thickness, and 
which must have required an enormous period for their 
accumulation; yet no one ignorant of this fact would 
have suspected the vast lapse of time represented by 
the thinner formation. Many cases could be given of 
the lower beds of a formation having been upraised, 
denuded, submerged, and then re-covered by the upper 
beds of the same formation,—facts, showing what wide, 
yet easily overlooked, intervals have occurred in its accu¬ 
mulation. In other cases we have the plainest evidence 
