CHAPTER Yll 
Influence of Volcanic Kocks on the Scenery of the Land — Efl'ects of Denudation. 
As considerable popular misapprehension exists respecting the part which 
volcanism has played in the evolution of the existing topography of the 
earth’s surface, and as the British Isles, from their varied geological 
structure, offer special facilities for the discussion of this subject, it may 
not be out of place to devote a final section of the present Introduction to 
a consideration of the real topographical influence of volcanic action. 
AVith modern, and especially with active, volcanoes we need not here 
concern ourselves. Their topographical forms are well known, and give rise 
to no difficulty. The lofty cones of the Vesuvian type, with their wide- 
spread lavas and ashes, their vast craters and their abundant parasitic 
volcanoes ; the crowded, but generally diminutive, cones and domes of the 
puy type, so well displayed in Auvergne, the Eifel and the Bay of Naples ; 
and the vast lava deserts of the plateaux, so characteristically developed in 
Iceland and Western America, illustrate the various ways in which volcanic 
energy directly changes the contours of a terrestrial surface. 
But the circumstances are altered when we deal with the topographical 
influence of long extinct volcanoes. Other agencies then come into play, 
and some caution may be needed in the effort to disentangle the elements 
of the complicated problem, and to assign to each contributing cause its own 
proper effect. 
Eeference has already been made to the continuous denudation of 
volcanic hills from the time that they are first erupted. But the com- 
parative rapidity of the waste aud the remarkable topographical changes 
which it involves can hardly be adequately realized without the inspection 
of an actual example. A visit to the back of Monte Somma, already alluded 
to, will teach the observer, far more vividly than books can do, how a 
volcanic cone is affected by daily meteoric changes. The sides of such 
a cone may remain tolerably uniform slopes so long as they are always 
being renewed by deposits from fresh eruptions. But when the volcanic 
activity ceases, and the declivities undergo no such reparation, they are 
rapidly channelled by the descent of rain-water, until the furrows grow by 
degrees wide and deep ravines, with only narrow' and continually-diininish- 
