of Devonshire and Corn •wall. 117 



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. 



Of the Moimtain-plain of Dartmoor Forest, 



When we trace up the courses of the rivers which flow through 

 Devonshire, we find they all rise in an ele\'ated 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 moun- 

 tain-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, whi<;h 

 opens at Ivy-Bridge. This little valley is at first contracted and 

 deep, with a rapid ascent. The general direction is nearly from 



