102 
VOLCANIC ROCKS AND SCENERY 
BOOK I 
The first point, then, which is established in an investigation of the 
topographical influence of old volcanic rocks is that their prevailing 
prominence arises from relative durability amidst universal degradation. 
When we proceed further to inquire why they vary so much from each 
other in different places, and how their complicated details of feature have 
been elaborated, we soon learn that such local peculiarities have arisen 
mainly from variations in the internal structure and grouping of the rocks 
themselves. 
Here again the general law of sculpture comes into play. The local 
features have depended upon the comparative resistance offered to the 
sculpturing agents by the different portions of a volcanic series. Each 
distinct variety of rock possesses its own characteristic internal structure. 
The lines along which atmospheric disintegration will most effectually carry 
on its carving w’ork are thus already traced in the very substance and 
architecture of the rock itself Each rock consequently yields in its own 
way to the processes of disintegration, and thus contributes its own dis- 
tinctive share to topographical feature. 
Among the massive rocks abvmdant examples of such special types of 
weathering may be cited, from the acid and basic series, and from superficial 
lavas as well as from intrusive bosses and sills. Acid bosses, such as those 
of granite, granophyre and quartz-porphyry, tend to weather into blocks and 
finally into sand, and as this tendency is somewhat unifoi'inly distributed 
through the I’ocks, they are apt to assume rounded, dome-sliaped or conical 
forms which, at a distance,' may seem to have smooth declivities, but on 
examination are generally found to be covered with a slowly-descending 
sheet of disintegrated blocks and debris (Fig. 346). When less prone to 
decay, and especially where traversed by a strongly-defined system of vertical 
joints, they may shoot up into tower-like heights, with prominent spires and 
obelisks. Basic bosses, when their materials decay somewhat rapidly, give 
rise to analogous topographical forms, though the more fertile soils which 
they produce generally lead to their being clothed with vegetation. Where 
they consist of an obdurate rock, much jointed and fissured, like the gabbro 
of the Inner Hebrides, they form exceedingly rugged mountains, terminating 
upward in serrated crests and groups of aiguilles (Figs. 331, 333). 
Acid lavas that have been superficially erupted weather into irregu- 
larly craggy hills, like the Hanks of Snowdon. Those of intermediate 
composition, where they have accumulated in thick masses, are apt to 
weather into conical forms, as may be seen among the Cheviot, Pentland 
and Garleton Hills (Figs. 109, 110, 133); but where they have been poured 
out in successive thin sheets they have built up undulating plateaux with 
teri’aced sides, as among the Ayrshire and Campsie Fells and the hills of 
Lome (Figs. 99, 107). Basic lavas have issued in comparatively thin sheets, 
frequently columnar or slaggy, forming flat-topped hiUs and terraced escarp- 
ments, such as are typically develojred among the Tertiary basalt-plateaux 
of the Inner Hebrides and the Faroe Islands (Figs. 11, 265, 283, 284, 286). 
One of the most frequent causes of local peculiarities of topography 
