ON THE DETRITAL TIN-ORE OF CORNWALL. 199 
bed of this rivulet* presents the undermentioned section ;t— 
(1). Gravel, sand, and mud ;— 
(2). Peat ;— 
(3). Roots, trunks, and branches of trees; mixed with quantities 
of nuts ;—and : 
(4). The tin-ground, which—although neither rich nor extensive— 
was wrought, near Saint Erth bridge by the use of much 
such a steam-engine as that worked at Marazion-march.t 
The easternmost of the two great granitic tracts in West 
Cornwall extends from Prospidnick and Nancegollan on the west 
to Ponsnooth and Budock on the east, and from near Polwheverell 
on the south to Wheal Buller on the north ; and—though separated 
at the surface by shallow scales of slate§—the range of Carn Brea 
and Carn Entral as well as the hill of Carn Marth are probably 
connected with it at inconsiderable depths. Both granite and 
slate are penetrated by broad dykes of felspar-porphyry (Elvan- 
courses) which in, rare instances, contain tin-ore.|| All these rocks 
are traversed by metalliferous veins (Jodes), which maintain an 
average direction of about 20° N. of E.—S. of W., but, in the 
very same mines, other (Cuunter) lodes bear nearly N.E.—S.W.** 
On and about the southern boundary of the granite tin-ore is their 
sole produce ;—towards the east, however, tin-ore prevails only 
near the surface, whilst the ores of copper occur beneath ;—and 
along the northern margin copper-ores abound at intermediate 
depths, but tin-ore is plentiful both above and belowt+} them. 
Numberless rills, rising far within the granitic region, unite to 
* The gravel, sand, and slime escaping during treatment of ores at the 
mines worked on various tributaries of this stream, have lately been collected, 
near the Saint Hrth Rolling-mills, and rewashed to some advantage. 
+ Mr. W. J. Rawlings, of Downs, near Hayle, MSS. 
“The largest lump of Jew’s-house-tin or Jew’s-bowl I have heard of 
‘* weighed 37lbs., * *; it formed part of a hedge in the Parish of Gwinear.” 
Carne, Cornwall Geol: Trans: ii, p. 293. 
+ Ante, p. 198. 
§ Thomas, (R.), Wining Review, No. viii, p. 265. 
|| Henwood, Cornwall Geol: Trans: v, p. 37-8, 85. 
q Ibid, v, p. 250; viii, p. 674. 
** (bid, V, p- 252. 
tt Jbid, v, Table li. Thomas, (J.), Journal of the Royal Institution of 
Cornwall, ili, pp. 191-2. 
