849 
M. Beudant an the Opals (^Hungary. 
It is a very remarkable circumstance, that petrified wood, in 
the state of opal, at least in the numerous specimens which I 
have seen from many different countries, never presents crystals 
of quartz in its interior, as is the case with wood in the state of 
woodstone, and which we find in the newest formations. It seems 
that, in the former case, the siliceous matter has been modified 
by some cause hitherto unknown, which has prevented it from 
crystallizing ; while, in the latter case, the molecules have been 
able to unite symmetrically, and to produce well defined crystals 
of quartz. 
Wood which has passed into a state of opal, occurs particu- 
larly in the pumice-conglomerates : it is sometimes in the form 
of trunks of great size, but more commonly in small pieces. It 
is difficult to say whether this wood has been silicified in the 
spots where it now occurs, or has been transported thither sub- 
sequently to its petrifaction. However, since opal-jasper exists 
in the same places in true kidneys, and since the trunks of wood 
which occur, have no appearance of being rolled, it is probable 
enough that the place where they are at present found, is that 
in which they have been impregnated with siliceous matter. 
Pieces of greater or smaller dimensions are also occasionally found 
even in fissures of the rocks where opal-jasper is deposited. 
I shall not mention any other localities of this opaliz^ed wood : 
it is to be found wherever opal-jaspers occur, that is to say, in 
all the places where it is formed by deposition from pumicose 
conglomerates. All the declivities of the trachytic groups which 
Hungary presents, have furnished it in greater or less quantity ; 
but the most remarkable locality is the village of Sajba, where 
the most beautiful varieties, whether for colour or for texture, 
are found. Almost all the specimens in collections have come 
from this place. 
From the details which have been given, we see that every 
where in Hungary, the opaline siliceous matters, whether pure 
or mixed, occur particularly in the trachytic conglomerates, or 
sometimes in infiltrations in the trachytic rocks themselves^ 
Every circumstance tends to confirm the remark, that this is 
the general position of these substances : in fact, they present 
themselves in the same manner at Mexico, according to the ob- 
servations of M. de Humboldt ; and in the same way we find 
