118 
THE AUTH OE EruOPfi. 
and small peiiis traversing beds of lignite and bituminous 
wood. Both these substances, so dift’erent in their origin, 
contain all that constitutes alum, that is to say, alumina, 
sulphuric acid, and potash. The ores of Tolfa, Milo, and 
Nipohgo ; those of Montioue, in which silica does not ac- 
company the alumina ; the siliceous breccia of Mont Dore, 
which contams sulphur in its cavities ; the alumiferous rocks 
of Farad and Beregh in Hungary, which belong also to tra- 
chytic and pumice conglomerates, may no doubt be traced 
to the penetration of sulphurous acid vapours. They are 
the products of a feeble and prolonged volcanic actioii, as 
may bo easily ascertained in the solfataras of Puzzuoli and 
the Peak of Teneriffe. The aliimite of Tolfa, which, since 
my return to Europe, I have examined on the spot, con- 
jointly with Gay-Lussac, has, by its oryctognostic characters 
and its chemical composition,’ a considerable affinity to 
compact feldspar, which constitutes the basis of so many 
trachytes and transition-porphyries. It is a siliciferous 
subsulphate of alumina and potash, a compact feldspar, 
with the addition of sulphuric acid completely formed in 
it. The waters circulating in these alumiferous rocks of 
volcanic origin do not, however, deposit masses of native 
alum, to yield which the rocks must be roasted. I know 
not of any deposits analogous to those I brought from 
Cumana; for the capillary and fibrous masses found in 
veins traversing beds of lignites (as on the banks of the Egra, 
between Saatz and Commothau in Bohemia), or efiiorescing 
in cavities (as at Freienwalde in Brandenburg, andatSegario 
in Sardinia), are impure salts, often destitute of potash, and 
mixed with the sulphates of ammonia and magnesia. A 
slow decomposition of the pyrites, which probably act as 
so many little galvanic piles, renders the waters alumife- 
rons, that circulate across the bituminous lignites and 
carburetted clays. These waters, in contact with carbo- 
nate of lime, even give rise to the deposits of subsulphato 
of alumina (destitute of potash), found near Halle, and 
formerly believed erroneously to be pure alumina, belong- 
ing, like the porcelain earth (kaolin) of Mori, to porplnwy 
of red sandstone. Analogous chemical actions may take 
place in primitive and transition slates, as well as in ter- 
tiary formations. All slates, and this fact is very important) 
