CHAP. VII 
LA WS OF EROSION 
lOI 
ing crests between them. It unchecked by any fresh discharge ot volcanic 
material, the degradation will at last have removed the whole cone. 
It is thus obvious that purely volcanic topography, that is, the terrestrial 
scenery due directly to the eruption of materials ii’oni within the eaith, can 
never become in a geological sense very old. It can only endure so long as 
it is continually renewed by fresh eruptions, or where it is carried down by 
subsidence under water and is there buried under a cover of protecting 
sediments. When, therefore, we meet with volcanic rocks of ancient date 
exposed at the surface, we may be quite certain that their present contours 
are not those of the original volcano, but ha^’e been brought about by the 
processes of denudation. 
It is true that, in the general erosion of the surface of the land, volcanic 
rocks of ancient date sometimes rise into wonderfully craggy heights, includ- 
ing, perhaps, cones and deep crater-like hollows, which to popular imagina- 
tion betoken contours left by now extinguished volcanic fires. Examples of 
such scenery are familiar in various parts ot Britain ; but the resemblance 
to recent volcanic topography is deceptive. There are, indeed, a few hills 
wherein the progress of denudation seems not as yet to have entirely 
removed the lavas and tuffs that gathered round the original vents. Some 
of the tuff-cones of eastern Fife, for example, present cases of this kind. 
Again, the great granophyre domes and ccjiies of tlie Tertiary volcanic series 
of the Inner Hebrides, though they have undoubtedly been extensively 
denuded, may possibly retain contours that do not greatly differ from those 
which these protruded bosses originally assumed under the mass of rock 
which has been removed from them. Nevertheless, putting such doubtful 
exceptions aside, we may confidently affirm that liills composed of ancient 
volcanic material give no clue to the forms of the original volcanoes. 
It can hardly be. too often repeated that the fundamental law in the 
universal deeay and sculpture of the land is that the waste is proportioned 
to the resistance offered to it : the softer rooks are worn down with com- 
parative rapidity, while the harder varieties are left projecting above them. 
As a general rule, volcanic rocks are more durable than those among which 
they are interstratified, and hence project above them, but this is nf)t always 
the case. No universal rule can, indeed, be laid down with regard to the 
relative durability of any rocks. While, therefore, topographic contours 
afford a valuable indication of the nature and disposition of the rocks below 
the surface, they cannot be relied upon as in all circumstances an infallible 
guide in this respect. No better proof can be offered of the caution that 
is needed in tracing such contours hack to their origin than is furnished 
by the old volcanic rocks of Britain. These eruptive masses, consisting 
usually of durable materials and ranging tlirough a vast cycle of geological 
time, usually rise into prominent features and thus support the general law. 
But they include also many easily eroded members, which, instead of 
forming eminences, are worn into hollows. They include, in short, every 
type of scenery, from featureless plains and rolling lowlands to craggy and 
spiry mountains. 
