Rich tin- 
mines. 
l8 8 NATURAL HISTORY 
fpecimen is, that it is. fixed on a bafe of a metallic hue at a a, b # , 
whofe compofition is micaceous, fcaly, and foft, like burnt tin ; it 
refills aqua forth , and as it has the exterior of native tin, might 
pofiibly be miftaken for fuch, though meer talc. 
Fig. xxi. A pentahedral pyramid of cryftalline tin, the ridges and 
fides concave, with a llender rhomboidal, well-finilhed plane on the 
apex ; it has many fhining yellow jlajnmulce in it, and not impro- 
bably holds gold. * 
xxii. A quadrilateral pyramid of tin, with a llender rhomboidal 
plane on one of the angles, with fome fparks of yellow as before ; 
but more refinous. , r c 
xxiii. A tin-grain extended like a book unfolding, lneie tour 
laft curious fpecimens came from Huel-fortune in Breag. 
xxiv. On a rhomboidal bafe two quatrilateral pyramids applied 
bafe to bafe. 
xxv. Two pentahedral pyramids joined bafe to bafe, each having 
a pentagonal cellule in the center of its apex. Both from St. Mewan 
Glebe. . ... 
xxvi. The fide- view of two hexagonal pyramids of tin joined 
bale to bafe. 
xxvii. One of the pyramids with the apex in front. 
xxviii. Section at the fradture of a piece of tin in undulated in- 
grailed Iheaths, nearly of an equal thicknefs throughout, inclofing 
one another ; the central nucleus is bafe cryftal. 
xxix. A white pyramidal grain of tin with tranfverle belts of 
folcick » 
xxx. A red, tetrahedral pyramid of tin found in a ftream-work 
in St. Auftel, 1757. . 
Let it be obferved, in the laft place, that whatever the figure oi 
thefe cryftals is, this is the pureft ftate of tin-ore, that ore produc- 
ing moft metal which has moft of thefe grains and largeft under 
any equal furface ; and black-tin is no more than thefe cryftals 
cleared of their impurities. 
The richeft tin-mine I have ever heard of, as to the quality of 
the ore, is one in the parifh of St. Agnes, near the beacon, called Pol- 
berou. Several parallel and contiguous veins, moftly oi large-grain 
cryftals, make the treafure of tin in fuch quantity, that, in the 
year 1750, they could not get horfes enough in the neighbourhood 
to carry the tin from the mine to the melting-houfe, but carried it 
in ploughs, a very unufual light (though doubtlefs a more effedlual 
and eafy draught where the ways will admit of wheels.) Great 
part of the ore was fo rich and pure that it needed not to be 
ftamped, and the lode is fo large that it affords vaft rocks of tin : 
one rock, in March 1750, was brought to Killinick melting-houfe 
near 
