ON THE DETRITAL TIN-ORE OF CORNWALL. 235 
disastrous consequences have been materially arrested, by planting 
the sand-hills with the sea-rush, (Arundo arenaria.)* 
(II). The mineral characters of the tin-grownd,—of the (Shelf) rock, 
on which it rests, and of the strata adjoining the Shelf, in various 
parts of Cornwall, are compared in Table I. 
(a). The earthy portions of the tin-ground. It seems that where 
—as in the moors of Towednack,t Wendron,{ Saint Austell,§ 
Luxulion,|| and Alternun§—the tin-grownd lies far within the 
boundary of its native series, the earthy ingredients of the one 
so closely resemble the general characters of the other as to 
forbid absolute identification of the rocks and veins from which 
the detritus had been reft. Even where the tin-grownd occurs near: 
the junctions of different systems—as in the swamps of Saint 
Austell,§ Luxulion,|| and Saint Columb-major,**—the débris of the - 
stratum immediately beneath always prevails ; sometimes, indeed, 
to, all but, an entire exclusion of the other.tt Generally speaking, 
however, the fragments are intimately mixed; but, perhaps, not 
always in the same proportions. This, however, is not always the 
case; for at Carnon the upper and the lower parts sometimes 
differ in mineral character.{{} 
(b). The metallic ingredients of the tin-ground comprehend 
several of the more plentiful ores ; viz :—blende,§§ copper in the 
* Paris, Guide to the Mount’s-buy, p. 162. Henwood, Cornwall Geol: 
Trans: v, p. 1138. 
4+ Ante, p. 196. 
¢ Ante, pp. 200-201. 
§ Ante, pp. 213-214, 
|| Anée, pp. 215-216. 
q Ante, p. 230. 
** Ante, pp. 217-218. 
++ ‘‘Le gite exploitable prés Pentovan...renferme des fragmens plus ou 
“moins volumineux...de toutes les roches et de toutes les gangues qui se 
“trouvent dans la contrée environnante, mais cependant a l’exception du 
“oranite.” HERON DE VILLEFOSSE, Richesse Minérale, ii, p. 354. 
tt “The bottom [of the ¢in-ground] has sometimes a different quality 
‘¢of waste associated with it as if deposited at a different period.” Tayzor, 
(C. Dyn), Proceedings of the Institution of Mechanical Engineers, (Cornwall 
Meeting), p. 161. 
§§ Henwood, Cornwall Geol: Trans: iv, p. 66. 
E 
