The ancients were not such fools as we take them to have been. They did condescend to consider the capabilities and the disadvantages of their building material before employing it. The "old men," when they constructed a wall of granite, always gave it two faces, and filled in with rubble between. By this means the rain did not drive through, although they did not employ mortar; and the ancient tenement houses on the moor are dry as snuff. But the modern architect insists on having the walls built throughout with lime, in courses, and the rain enters by these as by aqueducts. Then, to remedy the evil, the whole face of the house is tarred over or cemented, with what result to the prospect may well be conceived. The granite, though pervious, is so to a very limited extent when compared with limestone, and through a granite country there are no springs that issue from subterranean reservoirs. All the rain that falls on the surface runs off superficially, but not all at once, for on the granite lie enormous beds of peat, the growth and decomposition of moor plants through vast ages. These beds of peat are like sponges; they absorb the rain, retain it, and slowly give it up during the summer. In limestone districts the making of a river goes on within the bowels of the mountain, but in a granite district it takes place on its outside. Remove the beds of turf and peat, and there will be torrents after a shower, and then dry torrent beds.
To north and south of the equator of the moor lie vast tracts of bog in which the rivers are nursed, and without which they could not be. No visitor can realise what Dartmoor really is in the economy of nature as the mother of the Devonshire rivers till he has visited either Cranmere Pool, or the ridge on the south, where are the meres from which spring the Avon, the Erme, the Yealm, and the Plym.
The granite being of unequal hardness, its constituent crystals become separated by the action of the weather into an incoherent gravel, which in Cornwall is called growan. The process may be seen in full activity on any tor. Sometimes water lodges on a slab, and finding a soft spot begins to decompose it; then, when this is the case, the wind swirls the water about, and with it the grit is spun round and round, and this continues the work of disintegration, and finally a rock basin is produced.