OBSERVATIONS. 7 
* 
Killas, which might be juftly called 
Saxum . metalliferum cornubienfe> has been 
cl ailed by Mr. Kirvan, under the horn- 
f ** 
ftones } he found ioo grains of the grey 
lamellar killas to contain about 60 of 
filex, 25 of argill, nine of magnefia, 
and fix of iron. The greenifh fort con¬ 
tains more iron. Some of the cryftalli- 
zations of the Zinngraupen from Corn¬ 
wall are very remarkable. The moft 
regular, but which are found only rarely, 
are quadrangular prifms, with double 
quadrangular pyramids : there are fome 
alfo where the prifm is wanting, and 
both pyramids are joined together by their 
bafes, fo that the cryftal is odtoedral. 
Such regular cryftals are found at Tre- 
vaunance and Soil-hole, in the Darifh of 
St. Agnes. Similar quadrangular prif- 
matical cryftals, very tender, and often 
only of the thicknefs of a hair, are found 
in tin-ftone upon killas, at Polgooth, 
one of the rich eft tin mines, which at this 
time produces monthly a clear profit of 
B 4 from 
