ON THE DETRITAL TIN-ORE OF CORNWALL. 233 
The following pages—enumerate various relations between 
detrital repositories of tin-ore and the water-shed of Cornwall ;— 
show the respects in which their mineral characters either resemble 
or differ from, those of the (Shelf) rocks on which they rest, and 
of other strata in their several neighbourhoods ;—point out the 
mechanical conditions of the ¢in-grownd in different districts ;— 
describe the numerous deposits between the metalliferous beds 
and the surface ;—recapitulate their organic contents ;—and men- 
tion the unequal elevations at which tin-bearing detritus has 
been scattered. 
(1). The water-shed of Cornwall presents several remarkable 
flexures ;* but—except in two instances, one of small, and other 
of not very considerable; extent—its range is much nearer to the 
north, than to the south, coast. As the velocity of streams must be 
proportional to the inclination of their beds, those which have 
the same fall within the shortest distance, must—under ordinary 
circumstances—flow most rapidly ; whilst those which receive the 
drainage of the widest areas, must—under like conditions—attain 
the largest volumes. The small, short, shallow, and comparatively 
rapid brooks which fall directly into the Bristol Channel, thus 
possess proportionally greater degrading and transporting powers, 
than the longer, larger, and slower streams which flow into the 
English Channel. The former, therefore, deposit, on.their beds 
and banks, very little of the disintegrated matter, which they 
hold in suspension; but on reaching the sea-level, and there 
meeting the tide, their velocity is arrested, the solids subside, and 
form—at the mouths of the several estuaries—sand-banks and 
shoals which—as at Hayle, Padstow, and intermediate creeks © 
of less importance—materially- obstruct the navigation. The 
Camel—the longest of our northern rivers—can scarcely be in- 
eluded in this category ; for its direction is neither directly trans- 
_ verse, nor accurately parallel, to the water shed; but numerous 
important tributaries—flowing directly from the central range— 
materially modify its character. The streams which issue from 
the southern slope have, generally speaking, a longer and, con- 
* De la Beche, Report, p. 19, Fig. 1. Ante, pp. 193, 195, 197, 198, 199, 
201, 204, 212, 218, 215, 216, 219, 221, 225, 228, 280. 
