166 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
upon granite rocks than that of Dr. Geikie. He is speaking of 
Lochnagar, but in its degree the passage equally applies to 
Dartmoor. 5 
"Granite ... is remarkable for the perfection of its jointing, 
and generally for its toughness. It may crumble away on the 
surface, but otherwise may remain coherent and durable, though 
there are some varieties that decay far down into their mass. Its 
numerous joints, however, afford full scope for the action of frost. 
On lofty mountain crests, accordingly, granite frequently presents 
a most impressive array of splintered crags. [Our tors have a 
more mural character, caused by the horizontal and stratiform 
jointing, 6 perhaps due in part to the outward slip of the super- 
incumbent rocks.] Pinnacles and buttresses of the most varied 
forms and dimensions rise along the face of the precipices. Vast 
rifts, descending for several hundred feet, show where the joints 
have most easily opened, and naked vertical walls mark where the 
ice wedges, driven home by the winters of centuries, have at last 
detached huge slices from the face of the cliffs. . . . Inch by 
inch the vertical joints are . . . opened further into the face of 
the cliff. Along the edge one can, as it were, watch all the stages 
of the process, from the fine rift, just starting like a crack in a 
window-pane, up to the loosened pillar, which now stands gaunt 
and alone in front, and awaits the fate that is eventually to hurl 
it into the gulf below. Far down ... we can see the grey 
slopes cumbered with debris . . . avalanches of blocks . . . which 
in the course of ages have been wedged off from the cliffs and 
are now travelling slowly to the plains, still however a prey to 
frost and rain, sun and storm, and slowly breaking up into loose 
fragments as they descend " — the clitters of our tors. 
Then there are the milder and more constant influences of 
aerial and aqueous action combined : 
" After weeks of dry weather everything looks baked and dusty. 
The soil crumbles into powder at a touch. Each fitful gust of 
wind raises a cloud of dust from the roads, and blows away the 
sand that has been loosened on the surface of bare rocks. But 
the sky darkens, and at length rain descends. In a few minutes 
every channel on the roadways, every gully on the slopes, every 
runnel and watercourse, is the track of a muddy torrent, which 
sweeps down into the nearest brook. The brooks, swollen from 
bank to brae by the sudden descent of such innumerable tribu- 
taries, rush along laden with the fine particles of sand and disin- 
tegrated rock, Avhich they bear into the main stream of their 
5 Scenery of Scotland, pp. 39, 40. 
6 Hence what have been taken for artificial benches and tables on 
Crockern Tor. 
