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away over smooth surfaces, which would be actual planes if 
they were not channelled by innumerable river-valleys, we 
feel that denudation is indeed a great reality, and that the 
waste of this solid matter has been prodigious. 
The fact of such denudation being on all hands admitted, the 
only question for us to consider is, what are the agents which 
have brought about such marvellous results. Some maintain 
with much ingenuity — not to say special pleading — the 
greater effort of atmospheric over marine agencies ; others 
take a medium view, regarding both as having taken a part, 
and each special case as requiring the application of special 
agents. I shall now endeavour to point out one or two prin- 
ciples which, if kept in view, will, perhaps, serve as guides 
in the determining to which of these the configuration of the 
surface in every instance may be referred. 
Every one may for himself observe the manner in which 
rains, frost, and rivers work. The rain falls on the surface, it 
trickles down in gentle rills, then flows into deeper brooks, 
then into rivers, which, wearing down their beds and under- 
mining the banks, cause them to give way, and at the next 
flood the materials are carried away seaward. Such erosive 
action is clearly of a vertical kind, and results in the formation 
of channels more or less wide and deep, depending inter alia 
on the size of the river and the steepness of the fall. The 
action of all other agents of the air is in the vertical direction 
— tending to form channels and furrows, either branching or 
lying along parallel lines, as in the case of certain mountain 
chains. 
On the other hand, the wasting effect of sea waves has 
a character of its own which is strictly horizontal. The 
tendency of such wave-action along a coast line need scarcely 
be described. It has been fully ascertained that the destruc- 
tive effort is confined to the comparatively small depth to 
which the waves themselves extend, and that its tendency is 
to reduce to an uniform level all obstructions. The waves 
wear down the coast cliffs and rocks, spreading out the 
materials over the shore, or the currents carry them to more 
distant parts of the ocean. In general terms, therefore, we 
may state that the tendency of marine denudation is to the 
formation of level surfaces, such as plains and terraces, while 
that of atmospheric denudation is to produce hollows and 
channels ; in a word, each produces results peculiar to itself, — - 
the one being horizontal denudation, the other vertical. 
If these principles be admitted (and they can scarcely be 
disputed), I venture to think that many difficulties in deter- 
mining the nature of the eroding agents in each special 
instance will disappear., Wherever we find level surfaces or 
