118 THE ALTJAI OF EUEOF.E. 



and small reins traversing beds of lignite and bituminous 

 wood. Both these substances, so different in their origin, 

 contain all that constitutes alum, that is to say, alumina, 

 sulphuric acid, and potash. The ores of Tolfa, Milo, and 

 Nipoligo ; those of Montione, in which silica does not ac- 

 company the alumina ; the siliceous breccia of Mont Dore, 

 which contains sulphur in its cavities ; the alumiferous rocks 

 of Parad 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 action, as 

 may be easily ascertained in the solfataras of Puzzuoli and 

 the Peak of Teneriffe. The alumite 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 siliciferoua 

 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 efflorescing 

 in cavities (as at Ereienwalde 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 aa 

 so many little galvanic piles, renders the waters alumife- 

 rous, 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 subsulphate 

 of alumina (destitute of potash), fouud near Halle, and 

 formerly believed erroneously to be pure alumina, belong- 

 ing, like the porcelain earth (kaolin) of Mori, to porphyry 

 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, 



