Chap. IX. 
GEOLOGICAL EECOED. 
283 
process of degradation. The tides in most cases reach 
the cliffs only for a short time twice a day, and the 
waves eat into them only when they are charged 
with sand or pebbles; for there is good evidence that 
pure water can effect little or nothing in wearing 
away rock. At last the base of the cliff is under¬ 
mined, huge fragments fall down, and these remaining 
fixed, have to be worn away, atom by atom, until re¬ 
duced in size they can be rolled about by the waves, 
and then are more quickly ground into pebbles, sand, 
or mud. But how often do we see along the bases of 
retreating cliffs rounded boulders, all thickly clothed 
by marine productions, showing how little they are 
abraded and how seldom they are rolled about! More¬ 
over, if we follow for a few miles any line of rocky cliff, 
which is undergoing degradation, we find that it is only 
here and there, along a short length or round a pro¬ 
montory, that the cliffs are at the present time suffering. 
The appearance of the surface and the vegetation show 
that elsewhere years have elapsed since the waters 
washed their base. 
He who most closely studies the action of the sea on 
our shores, will, I believe, be most deeply impressed 
with the slowness with which rocky coasts are worn 
away. The observations on this head by Hugh Miller, 
and by that excellent observer Mr. Smith of Jordan 
Hill, are most impressive. With the mind thus im¬ 
pressed, let any one examine beds of conglomerate 
many thousand feet in thickness, which, though pro¬ 
bably formed at a quicker rate than many other de¬ 
posits, yet, from being formed of worn and rounded 
pebbles, each of which bears the stamp of time, are 
good to show how slowly the mass has been accumu¬ 
lated. In the Cordillera I estimated one pile of con¬ 
glomerate at ten thousand feet in thickness. Let the 
