CHAP, vn 
VOLCANIC SCENERY OF BRITAIN 
103 
among old volcanic rocks is the intercalation of very distinct varieties of 
material in the same volcanic series. Wliere, for instance, lavas and tuffs 
alternate, great inequalities of surface may be produced. ihe tuffs, being 
generally more friable, decay faster and give rise to hollows, while the 
lavas, being more durable, project in bold ridges or rise into mural escarp- 
ments (Fig. 265). Again, where dykes weather more readily than the rocks 
w’hich they traverse, they originate deep narrow clefts, while where they 
weather more slowly than the rocks around them, they project as dark ribs. 
Thus in Skye some dykes which rise through the obdutate gabbro are 
marked by chasms which reach up even to the highest crests ol the 
mountains (Fig. 333), while of those which run in the pale crumbling 
granophyre, some stand up as black walls that can be followed with the 
eye across the ridges even from a long distance. 
IVIany further illustrations of these principles might be cited here from 
the old volcanic districts of Britain. But they will present themselves 
successively in later chapters. For my present purpose it is enough to 
show that the scenery of these districts is not directly due to volcanic action, 
but is the immediate result of denudation acting upon volcanic rocks, 
modified and directed by their geological structure. 
It may, however, be useful, in concluding the discussion of this subject,^ 
to cite some typical volcanic regions in the British Isles as illustrations ol 
the relations between geology and topography, which, besides impressing the 
main lesson here enforced, may serve also to show some of the striking con- 
trasts which geology reveals between the present and former conditions of 
the surface of the globe. Among these contrasts none are more singuhir 
than those offered by tracts where volcanic action has once been rile, and 
where the picture of ancient geography presented in the rocks differs so 
widely from the scenery of the same places to-day as to appeal vividly to the 
imagination. 
The first district to which I may refer wdrere ancient volcanic rocks are 
well developed is that of Devonshire. The story of the Devonian volcanoes 
will be told in some detail in later chapters, when it will be shown that the 
eruptions were again and again renewed during a long course of ages. Tet, 
abundant as the intercalated lavas and tuffs are, they can hardly be said to 
have had any marked effect on the scenery, though here and there a hardei 
or larger mass of diabase rises into a prominent knoll or isolated hill. 
When the amount of volcanic material in this region is considered, we ma} 
feel some surprise at the trifling infiuence which it has exerted in the general 
denudation of the surface. 
To one who wanders over the rich champaign of southern Devonshire, 
and surveys from some higher prominence the undidating tree -crowned 
ridges that slope down into orchard-filled hollows, and the green uplands 
that sweep in successive waves of verdure to the distant blue tors of Dart- 
moor, the scene appears as a type of all that is most peaceful, varied and 
fertile in English landscape. In the trim luxuriance that meets the eye on 
every side, the hand of man is apparent, though from many a point ol 
