8 MINERALOGICAL 
from iooo to 1200 pounds fterling. The 
two cryftallizations which have been men¬ 
tioned are the bafes of all others, though 
thefe, by their extremities and angles 
being truncated, and by their being vari- 
oufly grown together, commonly appear 
very irregular. The fine polifh moft of 
thefe tin-cryftals have gives them a pecu¬ 
liar beauty, which is often heightened 
by a certain degree of tranfparency. Such 
tranfparent cryftals, in what is called 
Elvan, a Ipecies of argillaceous ftiifte, are 
found at Kreegbraws in Kenwyn. The 
rays of light pafling through gives them 
a bright brown-reddifh glofs, for which 
reafon the miners in Cornwall call them 
Rofin-tin. Tin cryftals of very glofiy 
appearance, but on account of a larger 
proportion of iron, of a quite dark black 
colour, are found at Poldice. 
Thefe fpecies of tin-ores give a pecu¬ 
liarly good tin, becaufe they are the moft 
free from arfenical pyrites. Among my 
fpecimens there are only a few which are 
mixed 
