200 ON THE DETRITAL TIN-ORE OF CORNWALL. 
form the river Cober, a principal feeder of the Loe-pool.* The 
swampy moorlands—portions of Carn-wartha, Mean Vroaz, 
Lezerea, Carth-vean, and perhaps of other, tenements—through 
which they find their way near Porkellis in Wendron have been 
wrought by Tin-streamers for many ages;t but, though their 
produce has greatly diminished, they are not yet quite exhausted ; 
in many cases, however, the principal detrital deposits of tin-ore 
are found to have been already explored by earlier workman. 
The upper part of Carn-wartha displays ;— 
(1). Angular and sub-angular masses of granite and of thin 
quartzose and schorlaceous veins mixed with lumps of 
peat and quantities of granite gravel and sand ;—the 
refuse of previous operationS............e-seeeeeee 12 feet ;— 
(2). Tin-ground. Quartzose, felspathic and schorlaceous sand 
and gravel, unequally sprinkled with, more or less 
rounded, granules of tin-ore ; interspersed, at intervals, 
with blocks of granite and schorl-rock, of much the 
same character as those in the overlying rubbish. This 
deposit, —now wrought for the first time—is ........ 12 feet thick ; 
The Shelf —of disintegrated granite ~ contains numerous small, isolated, 
bodies, and short, narrow, veins of quartz and schorl, irregularly impregnated 
with tin-ore. 
In Mean Vroaz great quantities of detrital tin-ore were 
obtained by the Streamers ; who—on reaching the granitic shelf—_ 
discovered the outcrop of lodes which were wrought, to some depth,} 
with considerable advantage. 
At Lezerea the successive deposits are,— 
(1). Peat; in the deeper part of which nuts and branches of 
hazel are sometimes found. The depth varies, but seldom 
CROWNS oo song sacccs bo obcocodoaoUoKDno DS daDUCdaCCN 4 feet ;— 
(2). Coarse granitic gravel containing, here and there, sub- 
angular stones of tin-ore ......- 0000000 cescecseees 2603 feet ;— 
(3). Granitic sand slightly, and at intervals, mixed with fel- 
spathic clay eetolevolaletatey cferereireret 25 5— 
(4). Tin-ground. Angular and somewhat rounded masses of 
. rather different character from that obtained at 
OhprmeWENOE sogcoacoasec00s lefetaphicntensleresae era eekane 3 5 SS 
The Shelf--of disintegrated, felspathic, granite—maintains a tolerably 
uniform composition to considerable depths; but in structure the shallower 
portions, which afford traces of detrital tin-ore, differ materially from the 
deeper, which are traversed by numerous small strings of tinny quartz-rock. 
* Rogers, (J. Jope), Cornwall Geol: Trans: vii, p. 352. 
+ Boase, Jbid, iv, p. 332. De la Beche, Report, p. 401. 
+ Mr. Frederick Hill, F.G.S8., of Penhellis near Helston, MS. 
