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SUB-AEEIAL DENUDATION 
of a long-ago age — a pressure that would seena to have set in in 
post- Carboniferous, pre-Permian times. By lateral pressure, 
indeed, were the strata raised, but by sub-aerial denudation 
they have been carved. The deep ravines and the broad 
valleys are alike the work of processes of earth-sculpture, 
effected in the main by rain and rivers. The vast amount of 
detritus carried down to the sea by rivers — -detritus which has 
contributed largely to the formation of the sand and mud-banks 
of the Severn estuary — is the waste matter removed during the 
process of sculpture, and adds not a little to the weight of 
other evidence in favour of the hypothesis of suh-aerial denuda- 
tion. In a word, just as Fire and Diluvium were the watch- 
words of the physical geologists of a past generation, so is 
Denudation the watchword of the physical geologist of to-day. 
2. — Suh-aerial Denudation, general and special. 
The agents of sub-aerial denudation may conveniently be 
divided into two classes ; first, those which exert their influence 
over the general surface of the country, such as rain, frost, and 
that more extended action which is known as weathering ; 
secondly, those which exert their influence along special lines or 
over special areas, such as rivers and streams and glaciers. We 
may call the work done by the first class of agents, general denu- 
dation, and that done by the second class of agents, special denu- 
dation. And we may say that the vertical contouring of the 
country — with which alone we are here dealing — is due to the 
differential action of general and special denudation on rocks of 
different poivet's of resistance. 
The action of general denudation may be likened to the 
action of sandpaper ; that of special denudation to the action of 
a file. If we take one of those not uncommon slabs of slate 
rock, through the midst of which there runs a band of much 
harder quartzite, and, choosing a flat surface, apply our file to 
