108  Cotn—The Rhyolites of the County of Antrim ; with a Note on Bauaite. 
Th. Parkmann* discussed the history of the subject, and concluded that there is a 
carbonate with the formula Al,0;.CO,, and that ‘the precipitate produced by 
the alkaline carbonates in solutions of alumina consists chiefly of this normal 
carbonate, generally mixed, however, with more or less of a more basic salt or 
hydrate of alumina.” MM. Urbain and Renoul + seem recently to have gone to 
the bottom of the matter, using alum and sodium carbonate, and also aluminium 
sulphate and ammonium carbonate. They conclude that the precipitate is a 
hydrated basic aluminium carbonate, equivalent to 2Al,0O;.CO,.8H,0; but 
carbon dioxide is given off from it at 30° C, and the material is ‘‘ un composé 
dune grande instabilité.” | Messrs. Morley and Muir, in their edition of Watts’ 
Dictionary of Chemistry,t still consider aluminium carbonate as of doubtful 
existence. 
MM. Urbain and Renoul’s unstable compound, exposed to slow natural agencies, 
might possibly break up, leaving aluminium hydrate, without such increase 
of temperature as was found necessary in the laboratory. But this increase is well 
within the range of climatic variations in Ireland ; and, all things considered, it 
is doubtful if any original basic carbonate could possibly have survived from 
the Eocene period to the present time. The process, as we may imagine it, is 
as follows :—Acid vapours attack the lavas in the early stages of their decay, 
forming soluble combinations of aluminium; secondly, sodium and _ potassium 
carbonates flow, in normal volcanic waters, over the deposited products (alums, 
&c.) resulting from the first attack; thirdly, the hydrated basic aluminium car- 
bonate thus formed is broken up again, losing carbon dioxide and some water, 
and leaving aluminium hydrate, combined with a portion of the iron oxide that 
has also undergone hydration. 
I do not picture this process as occurring in nature in large masses of 
aluminium salts. Round about volcanic vents, the products of solfataric 
action——sulphur itself, for instance-—may be found delicately permeating the 
whole groundmass of a lava, or all the interstices of an ash. From such deposits, 
hydrated aluminium carbonate might similarly be produced, resulting in aluminium 
hydrate in a finely divided state, mingled with the more normal products of the 
decomposing igneous mass. Processes of denudation, in carrying off the finer 
materials, will wash down the aluminium hydrate, or the aluminium iron hydrate, 
and will spread it out in some lake or hollow, with an admixture of clay, and 
perhaps of crystals, from the original rock. While the ready decomposition of 
basic rocks makes us regard them as the more probable sources of bauxite, we may 
*<¢Qn the Carbonates of Alumina, Glucina,”’ &c., Am. Journ. of Sci., ser. 2, vol. xxxiv. (1862), 
p- 326. 
}‘‘Sur une combinaison de l’alumine avec l’acide carbonique,” Comptes rendus, vol. Ixxxvili. 
(1879), p. 1188. t Vol. i. (1888), p. 696. 
