194, ON THE DETRITAL TIN-ORE OF CORNWALL. 
At the spot now wrought :— 
(1). Vegetable mould extends from the surface....to a depth of 2 or 3 feet; 
which, in some parts of the glen, is . 
succeeded by :— 
(2). Granitic gravel and shingle,* sparingly 
mixed with subangular masses of granite 
and its congeners containing thin strings 
of various vein-stones, sometimes 
sprinkled with tin-Ore;.......... atch » =~) +& few inches; 
ve however, is scarcely distinguishable 
rom— 
(3). The tin-ground, which also consists of 
granitic matter, subangular and rounded 
fragments of tin-bearing vein-stones, 
and pure tin-stone either more or less 
rounded or in angular fragments .... », 8 inches to 23 feet. 
The surface of the tin-ground maintains, throughout the ravine, a toler- 
ably uniform slope seaward; the rather considerable differences in its thick- 
ness being consequent on irregularities of contour in the granite (shelf) 
beneath. This varies little in composition, but much in hardness, within 
short distances; moreover in the softest portions the depressions (erosions ?) 
are deepest and most numerous, and—at the same time—the bed of tin-ore 
is thickest and best. + 
Large blocks of coarse-grained granite} occur at intervals ; 
* Pebbles of slate are mixed with gravel, shingle and boulders of granite 
in the cliff at and near Pornanvon. Bortase, Natural History, p. 76. Carne, 
Cornwall Geol: Trans: iii, p. 2830. Hxrnwoop, Jbid, v, p. 13. 
+ At Merry-meeting near Roche the stream-tin-ore is richest and most 
abundant where the granite beneath it is softest. Hrnwoop, /bid, iv, p. 61. 
t As the coarse texture, and open-jointed prismatic structure of the 
granite offer most favourable conditions to disintegrating atmospheric in- 
fluences, it is scarcely surprizing that this district contains no fewer than 
eleven logan-rocks ; viz., in :— sce 
Saint Levan, three. 
1 At Castle Treryn ........ The Logan-rock.......... Mentioned in every 
: _ Itinerary of the 
district ;— 
1 a5 near the base of the Cairn, a smaller one.. Undescribed :— 
1 At Bosistow-cliff ........ BRE oatoruiae coeeseceeee Described and fi- 
gured, (BLIGHT, 
Land’s-End, p. 
120) ;— 
Towednack, one. 
1 On Rosewall hill, ............0e0c0seeeeceeeceee.. Mentioned, (HEN- 
. woop, Cornwall 
Geol: Trans: v, 
p. 18) ;— 
