THE DARTMOOR VOLCANO. 
167 
drainage basin. And the rivers, dark with all this accumulated 
mud, sweep it downwards into the nearest lake or away out to 
sea." 7 
Before the formation of our present coast line ; before the days 
of our submerged forests, raised beaches, and bone caves ; before 
our Glacial epoch; before the Pliocene and Miocene and Eocene 
deposits originated; before the Chalk was formed, and the 
Oolitic building stones were laid down, and huge marine monsters 
thronged the Liassic seas — long before all these great stages in 
the world's physical history, the destruction of the great mountain, 
of which Dartmoor is but the stump, began. 
No doubt there were pauses in the operation. No doubt at 
times it was more rapid and again more slow. No doubt the sea 
has borne its part as well as wind and rain and ice and snow. 8 
But that the rocks of Dartmoor were exposed and broken down 
to form part of the materials of the beaches of the ancient 
Triassic sea of Devon, which once stretched at least from Sid- 
mouth to Plymouth, but have been themselves all but wholly 
7 Geikie, op. cit. pp. 20-1. 
8 Rain-wash has played a more prominent part in the modelling of the 
surface contours of the outer portions of the Moor than is commonly recog- 
nised. This is well seen at Blacket, where the Plymouth Lunatic Asylum is 
being built, on the flank of Ugborough Beacon. The granite in the hill 
immediately at the back of the asylum is moderately fine-grained, abounding 
in black mica, and porphyritic. Towards the surface it becomes loose textured 
and crumbly, and is seen (in a quarry section) to pass upwards into an 
incoherent mass of granitic sand, which apparently continues until the peaty 
subsoil is reached. Further investigation shows, however, that the upper 
part of this sandy stratum is distinguished from the lower by colour-bands 
indicating a quasi-stratification generally dipping with the hill, but with 
irregularities which suggest the filling up of hollows ; and that this stratified 
material contains large angular fragments of rock. These are absent from 
the loose stuff below, in which we recognise disintegrated granite pure and 
simple. All is granite sand throughout ; but while the lower portion is in 
situ, the upper has been gradually washed down the hill, and collected in 
gullies or hollows — or, where the conditions are favourable, in sheets — to a 
thickness ranging up to four feet — the solid rock being denuded on the higher 
slopes of the tor to a similar extent. The quarry where these observations 
were made is close to the junction of granite and hornblendic slate ; and at 
one point this granitic rain-wash is seen to overlie a subsoil of angular 
fragments of slate in clay formed by the breaking up of the outcrop of the 
schistose rocks, and itself showing traces of " hill creep." In more favourable 
localities the results of such subserial denudation may be very important, so 
that we need not always call in the agency of constant streams or ice-caps to 
account for extensive detrital deposits at hill bottoms. 
