THE DARTMOOR VOLCANO. 
149 
The Dartmoor heights are mainly connected ranges of high 
land, with a general run north and south. There are only two 
points where a contrary tendency is strongly marked. South of 
Fur Tor and Cut Hill a ridge runs irregularly east and west from 
the valley of the East Dart to that of the Tavy. In the southern 
quarter a semicircle of high ground sweeps round Fox Tor Mire 
eastward and southward, and extends westward toward Cramber 
Tor. The heights are more continuous in the central portion of 
the Moor, east and west of which there are many isolated hills. 
The fall in the upper waters of the Dartmoor rivers is very 
considerable, and if their volume in these regions were any way 
proportionate (only the case in very rainy weather), their ex- 
cavating work would be enormous. The two branches of the 
Dart, to Post Bridge and Two Bridges respectively, fall about 
150 feet a mile; thence to Dartmeet and to the edge of the 
Moor at Holne, 80 feet. Holne Bridge is just 200 feet above 
datum. The Teign gives much the same results. The Taw and 
the Okements fall still more rapidly — from 180 to 200 feet a 
mile. The Tavy, between its source and Tavistock, averages 
about 140 feet a mile ; from Tavy Cleave onward about 90. The 
Walkham falls at the rate of 160 feet a mile to Merivale Bridge, 
thence to the confines of the Moor about 80. The Plym to 
Shaugh Bridge falls some 160 feet a mile. The Meavy about 
120-150 between the source and the Head Weir, 90 thence to 
Shaugh Bridge. 
The sudden drop from the inferior outskirts of the Moor to 
the bordering lowlands is strikingly shown in the ravines through 
which the chief streams descend. The Plym, at the Dewerstone ; 
the Meavy, at Yannadon ; the Walkham, at Huckworthy ; the 
Tavy, at Tavy Cleave; the Lyd, at the Gorge; the West Okement, 
at Meldon ; the Taw, at Belstone Cleave ; the Teign, at Fingle 
Bridge ; the Bovey, at Lustleigh Cleave ; the Dart, at Holne ; 
the Erme, at Ivybridge ; the Yealm, at Awns and Dendles. All 
these tell one tale, as to which we shall have more to say anon. 
Such are the chief physical characters of modern Dartmoor. 
The key to the origin and geological history of Dartmoor is 
the nature of the rock of which it is mainly formed, to which 
its other physical conditions are subordinate. The idea that 
