10 GRANITIC ROCKS OF CORNWALL. r Ch. II. 



the former sometimes possess a good soil, adapted either for 

 tillage or pasture. 



The primary schistose districts which surround the granitic 

 rocks partake of the same characters : they are, however, in 

 general, more capable of cultivation ; and their valleys find a 

 better drainage in the rivers, by which they are frequently 

 traversed. 



When we take a bird's eye view of a primary country, the 

 surface very commonly exhibits a system of valleys, which 

 run parallel with the central ridge of granite ; and these 

 longitudinal valleys are intersected by others which cross 

 them at right angles ; and as the intermediate hills are more 

 or less rounded, the surface of the country has an undulating 

 appearance in both directions, which has been often and aptly 

 compared to the waves of the sea ; and the simile is farther 

 appropriate, inasmuch as, when the curves of the hills are 

 regular and gentle, or variously contorted and abrupt, they 

 resemble the sea when agitated by the wind with different 

 degrees of violence. 



On the form of these two systems of valleys many features 

 of the primary districts depend. Through them the rivers 

 flow, seeking an outlet into the sea by the nearest continuous 

 descent : sometimes they effect this along the longitudinal, 

 at others, through the cross valleys, receiving tributary 

 streams on either hand from the lateral valleys, or curved 

 hollows, which they intersect. But very often the course of 

 a river is diverted more than once from its original valley, in 

 consequence of the concavity of a cross curve rising above its 

 level, or that of the other system descending below this point, by 

 which irregularity the stream flows along a more favourable 

 drainage ; and, not unfrequently, when both systems of valleys 

 oppose like obstructions, the rivers, thus dammed up, are 

 converted into lakes ; which again obtain an outlet, or not, 

 according to the positions of the adjacent concave curvatures 

 of the intersecting ranges. In this manner, the course of a 

 river, whilst within a primary region, is determined by the 



