838 M. Beudatit o/J Opals (}f‘ Hungary, 
in fissures ; arid it follows that the opal itself, which is deposited 
in the sairie manner, also contains it in greater or less quantity. 
Thus, pyrites, which appears to belong to the white sulphuret 
of iron, readily decomposes oil exposure to the air, and affords 
sulphate of iron : it sometiiries causes the decomposition of the 
rock, and even of the opal, in which it is contained. According 
to my observation, this substance is rather rare, and occurs only 
here and there, especially in the dark-coloured and soft parts of 
the matrix {gangue). I remember to have heard the existence 
of pyrites in the opals of Hungary denied. It was asserted, that 
it only occurs in the varieties of this substance, which are met 
with in certain metallic veins ; but the observations which I have 
made at Cservenicza, and the specimens which I have brought 
from thence, place beyond doubt what has already been said by 
M. Mohs, with regard to this subject. {Leonard's Tasclieiu 
huch^ 7. annee^ p. 296.) 
Cavities of the porphyritic rock sometimes occur, filled with 
opals of different kinds, in very thin laminae, which are very nu- 
merous, and separated from one another: these laminae are 
sometimes arranged in a paiallel manner, and sometimes cross 
each other at various angles. The aggregate of these laminae 
forms a small mass, which has the appearance of being cracked, 
and which reminds us of the structure of certain nodules of 
quartz, which sometimes occur in veins, to which the name of 
Hackly Quartz {Qiiarz liache) has been givdn. 
It is not necessary for me to discuss, in this place, the nature 
of opal. I shall only mention, that analysis has only discovered 
silica and water, in the proportion of 1 to 8 or 10. Some che- 
mists have considered this substance as a hydrate qf silica ; but 
others think that the water is merely interposed : such is the opi- 
nion of M. Berzelius, and is founded principally on the fact, that 
silica cannot be made artificially to combine with water. I can- 
not speak decidedly in favour of either of these opinions ; but I 
may observe, that it is evident that opal, as well as all that has 
been designated by the name of Silex Resinite, differs essentially 
from hyaline quartz, and cannot be considered as being to this 
substance, what the compact carbonate of lime is to the crystal- 
lized carbonate. Opal is very fragile ; its fracture in the purest 
and most transparent parts, is in no respect that of quartz ; it 
