6 MINERALOGICAL 
irregular and compadt ipecies are called 
fo. The cryftallized tin-ftones, on the 
contrary, are called Zinngraupen , if the 
cryftals are diftindt, and fomewhat large j 
but Zinnzwitter, if the cryftals are 
I 
finaller, and not fo diftindt:, refembling 
fmall grains, fcattered through a com¬ 
pact raw tin-ftone, or a ftone of any 
other kind. 
Thefe fpecies of Cornifti tin-ores dif¬ 
fer from thefe of Bohemia and Saxony 
principally in this, that in general they 
contain lefs iron and lefs arfenic; and this 
Is the caufe of the preference the Eng- 
lifti tin in general obtains, on account of 
its purity. 
The common matrix is Killas and 
Growan. A defeription of both is to 
be found in Mr, Kirvan’s mineralogy, 
Growan confifts of white clay, mixed 
with mica and quartz, and is of no pe¬ 
culiar texture ; it is, therefore, nothing 
elfe but decayed granite, in which the 
feldfpat has been broke down into clay, 
Killas* 
