Page:A Book of Dartmoor.djvu/40

 representation of the idol was given in the Transactions of the Devonshire Association for 1875. The new Plymouth Reservoir overlies an old lakebed. Taw Marsh was also once a sheet of rippling blue water, but the detritus brought down in the weathering of what once were real mountains has filled them all up. Dartmoor at present bears the same relation to Dartmoor in the far past that the gums of an old hag bear to the pearly range she wore when a fresh girl. The granite of Dartmoor was not well stirred before it was turned out, consequently it is not homogeneous. Granite is made up of many materials: hornblende, feldspar, quartz, mica, schorl, etc. Sometimes we find white mica, sometimes black. Some granite is red, as at Trowlesworthy, and the beautiful band that crosses the Tavy at the Cleave; sometimes pink, as at Leather Tor; sometimes greenish, as above Okery Bridge; sometimes pure white, as at Mill Tor. The granite is of very various consistency, and this has given it an appearance on the tors as if it were a sedimentary rock laid in beds. But this is its little joke to impose on the ignorant. The feature is due to the unequal hardness of the rock which causes it to weather in strata. The fine-grained granite that occurs in dykes is called elvan, which, if easiest to work, is most liable to decay. In Cornwall the elvan of Pentewan was used for the fine church of S. Austell, and as a consequence the weather has gnawed it away, and the greater part has had to be renewed. On the other hand, the splendid elvan of Haute Vienne has supplied the