,37 
Dr. Eqfcn on Cryjlallization. 
fbfteft of the whole clafs to which it belongs. * 
Had thefe fa< 51 s been fufficiently attended to, the 
component parts of precious ftones had probably- 
been better underftood, and the different figures 
of thefe fubftances might have been Ihewn to 
have proceeded from one conftant and permanent 
law of cryftallization. 
But, be this as it may, the principles already 
in our poffeflion are fufficient to fupport a 
rational theory, not only of the formation of pre¬ 
cious ftones, but alfo of thofe fubftances known 
by the name of pebbles. Many fubftances which, 
under proper circumftances, Ihoot into cryftals 
of the moft regular form, concrete into diforderly 
mafies, when the procefs is carried on with too 
much precipitation. Have we not reafon there¬ 
fore to conjecture, that pebbles, agates, and even 
common flint are the produfts of fome fuch irre¬ 
gular cryftallization ? This may be exemplified 
in the pebbles found among the rocks of 
Arthur’s feat near Edinburgh, which are a vitri- 
fiable matter, mixed with different heterogeneous 
fubftances, from which a variety of colours and 
irregular fhapes is produced - 3 and the rocks, 
* The topaz feems to be an exa£l rock cryftal with a 
yellow colour, probably from the calx of fome metal; it is 
a prifm of fix irregular fides. I have feen fome of them of 
3 very large fize; many are found in the mountains of 
Scotland, and, fometimes, at the foot of mountains, buried 
jn the earth, having been wafhed down by the rains. 
P 3 where 
