240 : ON THE DETRITAL TIN-ORE OF CORNWALL. 
even of this, there is little doubt but that more has found its way 
to the crucible than to the cabinet. 
. 
(III). Mechanical conditions of the tin-ground. Every earlier 
writer* and all preceding details| show that the largest blocks in 
all these deposits bear marks of, greater or less, abrasion and 
attrition. In the immediate vicinity of their original localities, it 
is true, they are sometimes subangular or but slightly rounded,{ 
but within distances comparatively short rude globular and 
spheroidal forms prevail. Of the more comminuted ingredients a 
principal portion is, perhaps at least, equally rounded ; the rest 
comprehends angular fragments of various rocks and veins, perfect 
as well as fractured crystals of tin-ore; and with these any gold 
present is usually mixed. All interstices between the rougher 
components are filled with sand and clay.§ 
At Levrean|| the tin-grownd is divided horizontally, by— 
(a). False-shelf which has extended through the entire tract 
wrought during five and forty years past, and yet exists in the 
present works. It consists of angular and subangular masses of 
granite, imbedded in disintegrated granitic matter; scarcely dis- 
tinguishable from the tin-ground above and below; save that it 
contains but few masses of veinstone, and is destitute of ore. 
Where the tin-ground is thus divided, by false-shelf, the lower 
is usually the richer portion. 
(IV). The bed which rests on the tin-grownd might scarcely have 
been distinguishable from the detritus beneath; but—that its 
ingredients are, perhaps, rather less abraded ;—that, in one part 
or other of every detrital district, it contains masses of barren 
rock incomparably larger than any contained in other parts of the 
* Ante, pp. 193—192. 
+ Ante, pp. 193—232. 
+ Ante, pp. 197, 217, 231. 
§ In one spot, at least, flints have been obtained from this part of the 
series. Ante, p. 215. 
|| Henwood, Cornwall Geol: Trans: iv, p. 62; Ante, p. 214. 
q False-Shelf has been observed also at the Merry-meeting, and Grove, 
stream-works in Saint Austell, at Broad-water in Luxulion, at Chyvenhall in 
Paul, and, perhaps, also at Carnon in Feock. Hrnwoop, Cornwall Geol: 
Trans: iv, pp. 59, 61, 62, 64. Carne, Jdid, p. 104. 
