246 ON THE DETRITAL TIN-ORE OF CORNWALL. 
western declivities ; but whether alike plentiful on opposite sides 
of the water-shed* seems unknown. 
It has been shown} that a slightly elevated body of ‘slate 
extends from the northern slope of Hensbarrow and Killivreth 
Down,—surrounds the granite of Castle an Dinas and of Belovely 
Renwom, —and merges in the schistose strata which border the 
Bristol Channel. North-east of this elevation both the rocks and 
the—more or less—rounded detritus in the moors of Saint 
Austell and Lanlivery are almost exclusively granitic ;{ south- 
west of the dividing range, however, the shelf, the tin-grownd,.and 
the overburden are for—by far—the most part of slate and elvan, 
mixed, at intervals, with small proportions of granite,§ all bearing 
traces of abrasion. But whilst the band of schistose rocks main- 
tains a higher level than the dissimilar—though it may be con- 
temporaneous—beds of detrifus on either side, it is much lower 
than the peaks, ridges and slopes of Hensbarrow, Killivreth 
Down, Helmen-Tor, Belovely Beacon, and Castle an Dinas; from 
the rocks,|| Jodes, and thin strings of vein-stone, from some of, if 
not from all, which—small as their produce has been of late years 
—both the tin-grownd and the overlying débris are assumed to have 
been derived,f whilst this boundary of slate, between the 
different kinds of transported matter, is, itself, free from all trace 
of detritus. Moreover the rather considerable tract of cultivated 
land which surrounds the village of Tregoss and the hamlet of 
Pendean,—though bounded on three of its sides by the refuse of 
ancient stream-works**—hbears no specimen of drift, or evidence of 
diluvial action.+t 
* Ante, pp. 233—235. 
+ Ante, pp. 212, 218. 
+ Ante, pp. 213—216. 
§ Ante, pp. 216—218. 
|| Boase, Cornwall Geol: Trans: iv, p. 252. Henwood, Jbid, v, p. 120, 
Note. Ante, p. 212. 
q ‘Those who have studied the decomposed granite near Saint Austell, 
‘traversed as it is by a multitude of branches and strings of oxide of tin, 
‘would have little difficulty in perceiving that if a body of water were made 
“to rush over it, the decomposed granite would be readily removed, and that 
“the broken-up strings and branches of tin-ore would be rolled into pebbles, 
“and distributed just as the stream-tin now occurs down the valleys in the 
“neighbourhood.” Dr ta Brecue, Report, p. 398. 
** Ordnance Geological Map, Sheet xxx. 
++ Boase, Cornwall Geol: Trans: iv, p. 248. Ante, p. 218, Notef. 
