Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/129

 Rh as the point where the high road to Tavistock joins, the grauwacke-slate continues. Its stratification is very distinctly seen at the passage of the Tamar near Calstock, and of the Tavy near Beer Ferris.

At Calstock, the strata are cut more abruptly on the left bank, and at Beer Ferris on the right bank, from which we may infer, that the depth of the Tamar and the Tavy is not the same at both banks, the depth of a river being in general increased as its banks become more precipitous.

The grauwacke-slate also continues in the road from Plymouth to Ivy-bridge. On approaching the latter place we find pebbles and even adventitious blocks of granite, which being brought down into the plains, by the rivers which flow from the high land of Dartmoor, shew that that district is formed of primitive rocks.

When we trace up the courses of the rivers which flow through Devonshire, we find they all rise in an elevated and extensive plain situated nearly in the middle of the county, and upon which the adjacent rocks, gradually rising as they approach it, are found to rest. The south and north sides are the water-sheds of the mountain-plain. The Tavy, the Plym, the Yealme, the Erme, the Avon, and the Dart, flow down the southern side; the two Oakments and the Taw run to the north: there is only the Bovey on the south-east, and the Lyd on the west, and these are both very small streams.

I entered Dartmoor forest, by the valley of the Erme, which opens at Ivy-Bridge. This little valley is at first contracted and deep, with a rapid ascent. The general direction is nearly from