292 
IMPERFECTION OF THE 
>Chap. TX. 
Geology, and have been surprised to note how author 
after author, in treating of this or that great formation, 
has come to the conclusion that it was accumulated 
during subsidence. I may add, that the only ancient 
tertiary formation on the west coast of South America, 
which has been bulky enough to resist such degradation 
as it has as yet suffered, but which will hardly last to a 
distant geological age, was certainly deposited during a 
downward oscillation of level, and thus gained consi¬ 
derable thickness. 
All geological facts tell us plainly that each area 
has undergone numerous slow oscillations of level, and 
apparently these oscillations have affected wide spaces. 
Consequently formations rich in fossils and sufficiently 
thick and extensive to resist subsequent degradation, 
may have been formed over wide spaces during periods 
of subsidence, but only where the supply of sediment 
was sufficient to keep the sea shallow and to embed and 
preserve the remains before they had time to decay. 
On the other hand, as long as the bed of the sea re¬ 
mained stationary, thieh deposits could not have been 
accumulated in the shallow parts, which are the most 
favourable to life. Still less could this have happened 
during the alternate periods of elevation; or, to speak 
more accurately, the beds which were then accumulated 
will have been destroyed by being upraised and brought 
within the limits of the coast-action. 
Thus the geological record will almost necessarily be 
rendered intermittent. I feel much confidence in the 
truth of these views, for they are in strict accordance 
with the general principles inculcated by Sir C. Lyell; 
and E. Forbes subsequently but independently arrived 
at a similar conclusion. 
One remark is here worth a passing notice. During 
periods of elevation the area of the land and of the 
