M. Beudant m the Opals of Hungary, S$§ 
resOTbles the fracture of dried starch, both in the form of it& 
fragments, and in the lustre. Opal is easily scratched by quartz ; 
it is lighter, and the pearly lustre which it assumes . on being 
heated, — the opacity which it acquires, — the facility with which 
it may then be crumbled down, — the kind of decomposition 
which it undergoes, — all these circumstances indicate a peculia- 
rity of nature, which, however ill determined as yet, is not the 
less evident. 
It has been erroneously asserted, that opal occurs in Hungary, 
in successive beds, of many feet in thickness. In all places 
where I have observed this substance, I have always found it in 
nests, or rather in veins of greater or less extent, extending in all 
directions. It is in this manner that it is found at Cservenitza, 
where this substance is very abundant, and has evidently fiUed 
either open fissures in the rock, posteriorly to its consolidation^ 
or irregular cavities naturally existing in it. Not unfrequently, 
the fissures still open, have their walls covered with a lining of 
opal of greater or less thickness : in other places, the cavities or 
cellules filled with stalactites, evidently announce a substance 
which has been infiltrated ; lastly, the veins, which often present 
vacuities, in which the matter is raamillated, and in some mea- 
sure purified, still lead to the same result. On examining the 
specimens, we often perceive the fissures of the rock by which 
the substance of ^ the opal has penetrated into the cavities which 
it has filled in part or in whole. In other cases, the manner in 
which the small nests are disseminated in the matrix, seems to 
indicate that they have been formed simultaneously with the 
rock which includes them, and they recal to mind the kidneys 
of flint which have been formed in our chalks. 
The rock in which the nests and veins of opal occur at Cser- 
venitza, has given rise to many opposite opinions ; some have 
considered it as a production of fire others, as of aqueous ori- 
gin. Hence it has been named, sometimes a lava^ sometimes a 
granite^ or rather a porphyry^ an altered porphyry^ a hrecciated 
porphyry a hardened clay. The fact is, that the mass of moun- 
tains in which this beautiful substance is found, is entirely form- 
ed of trachytic conglomerates^ which extend from the heights of 
Sovar to that of Kaschau, and , which present a great number of 
varieties, according as the fragments of trachytes have been more 
