[CHAPTER X.]
DARTMOOR AND ITS ANTIQUITIES
Geological structure of Dartmoor—Granite—"Clitters"—Building with granite—The bogs—The rivers—Rock basins—Logan stones—Kaoline deposits—Hut circles—Cooking-stones—Pottery—Pounds—Grimspound—Position of women in early times—Approximate period to which the relics belong—The cromlech—The kistvaen—The stone circle—The stone row—The menhir—Cairns—Modes of interment among the pagan Irish—Stone crosses—Tinners' burrows and stream works—Blowing-houses.
The great irregular tableland of Dartmoor, an upheaval of granite over a thousand feet above the sea, and in places attaining to above two thousand, occupies two hundred and twenty-five square miles of country. Of that, however, less than one half is the "Forest" and belongs to the Duchy of Cornwall. Around the forest are the commons belonging to the parishes contiguous to the moor.
The moor is almost throughout of granite. At the outskirts, indeed, gabbro and trap exist, that have been forced up at the points where the granite has burst through the slate, and these later uprushes of molten matter have greatly altered the granite in contact with them, and have produced an elvan.
The most extraordinary difference in kinds and composition exists throughout the granitic area. Some granite is very coarse, full of what are locally called "horse-teeth," crystals of felspar, other is finely grained. Some is black with schorl, some, as that of Mill Tor, white as statuary marble. Granite was not well stirred before it was protruded to the surface. The constituents of granite are quartz, felspar, and mica; the latter sometimes white, at others usually black and glistering. The felspar may be recognised as being a dead white. The black shining matter found near where are veins of tin, is schorl.
It is the opinion of modern geologists that the granite never saw daylight till cold and consolidated, and that granite when in fusion and erupted to the surface resolves itself into trap. The pressure of superincumbent beds prevented perfect fusion. In its altered condition when perfectly fused it may be seen in Whit Tor, near Mary Tavy.
But, it may be asked, what has become of the beds that overlay the granite? They have been washed away. In Exmoor we do not meet with the granite. It had heaved the slates, but not sufficiently to so dislocate them as to enable the rains and floods to carry them away and reveal the granite below. If Dartmoor granite could but have retained its covering matter, the region would have been indeed mountainous. In Shavercombe, a lateral valley of the upper Plym, may be seen traces of the original coverlet of slate, much altered by heat.
The granite looks as though stratified, but this is deceptive. It is so unequally mixed that some flakes or layers are harder and more resistant to atmospheric forces than are others, and where the granite is soft it gives way, presenting a laminated appearance. Moreover, the granite is full of joints. Where these joints are vertical and numerous, there the rocky masses break into fragments. Bellever Tor is a good instance. This imposing mass looks as though, when rising out of the Flood, it had shaken itself, like a poodle, to dry itself, and in so doing had shaken itself to bits. Lustleigh Cleave is another instance. Every tor is surrounded by a "clitter" (Welsh clechir), and these clitters are due to the disintegration of the granite in horizontal beds, and then on account of their joints horizontal and diagonal, falling into confused heaps. Where the joints are not numerous and not close together, there the rocks cohere and form tors. In many, as Vixen Tor and Mis Tor, the pseudo-bedding lines are very distinct. Where the soft beds are infrequent, there the granite forms great cake-like blocks as in Hey Tor. The tors are, in fact, the more solid cores as yet not overthrown by natural agencies. Such a core is Bowerman's Nose, and around it is the "clitter" of rock that once encased it.
Granite is very pervious to water, as everyone knows who lives in a house built of it by modern architect and masons.