Subterrancan Erosion, and some 
of its effects. 
BY OW... 5 HONE, .. BOGS. 
Read January 7th, 1892. 
N December gth, 1891, I read a paper before the Geological 
Society of London, entitled ‘‘ Subterranean denudation, &c.” 
In the discussion which followed, there was a consensus of 
opinion that Subterranean evoszon was the better definition of the 
two. It was argued that denudation was the act of laying dare, 
and could not therefore be subterranean. ‘The objection to the 
word denudation was too obvious to be disregarded, though, I 
confess, the substituted word erosion is apt to lead to mis- 
understanding between suderial erosion and subferrancan erosion 
if we do not clearly comprehend that the former works from 
without inwards, and the latter works from wzfhin outwards. 
Subterranean erosion has consequently nothing to do with the 
undermining from without-inwards of our coasts by the action 
of the sea—or inland that of rivers—along their banks. 
SUBTERRANEAN EROSION. 
Subterranean erosion may be either chemical or mechanical 
in its action. The removal 7” soluéion of rock material from 
beneath the surface by springs is too well-known to need any 
further comment. The matter removed is, however, so 
infinitesimal in bulk as to require a great lapse of time before 
it can produce any perceptible physical effects. 
MECHANICAL SUBTERRANEAN EROSION. 
It is very different with the mechanical action of subterranean 
erosion, We must bear in mind that water cannot remove any 
solid matter of greater specific gravity than itself, unless it 
forces it along an inclined plain. 
The mechanical power of subterranean erosion must vary with 
the angle of inclination along which the underground water 
flows—the character of the strata through which it moves, and 
the hydraulic pressure or “head” which impels it forward 
towards the point of escape. With such varying factors we may 
expect very divergent effects. All other things being equal— 
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