310 | M. Daubrée on the © 
may likewise be easily obtained by the reaction of lime on 
the chloride of magnesium, which is found among the abun- 
dant chloriteous vapours of the fumeroles of Vesuvius. The 
same chloride, decomposed by steam, likewise gives periclase, © 
and the chloride of zine furnishes owidated zine crystallized. 
The results which have been indicated, lead to geological 
consequences which can be referred to here only very briefly. 
Ido not mean to affirm as a fact, that all the silicates, which 
compose the mass of crystalline rocks, are formed by vapours. 
But even among the melted rocks of Vesuvius, a certain 
number of minerals are found, to which M. Scacchi has re- 
cently drawn attention, and which appear to be the products 
of sublimation. 
Among the minerals of the most ancient formations, there 
are likewise many which could not come, in the way of fusion, 
to line the fissures where these minerals are now met with so 
much insulated ; such as the pyroxenous diopside with garnet 
of Piedmont and the Oural, the adulaire and pericline felspars 
of the Alps, the epidotes and axinites of the Oisans, and 
many others. 
The privileged richness of the crystalline limestones in 
minerals, often strangers to the neighbouring rocks, cannot 
result solely from the circumstance that the lime, reacting 
in them on the silica, has formed particular silicates. What- 
ever may have been the original impurities of the limestones, 
corundum, spinelle, periclase, and chondrodite, could not be- 
come developed in them, without the subsequent introduction 
of chemical agents which were foreign to them. | 
_ All the varied productions of transport, silicates, alumi- 
nates, oxides, and other combinations, whether formed in fis- 
sures, or in the heart of rocks now become very compact, 
appear to me to be explained in the most satisfactory manner 
by the intervention of emanations of chlorides and fluorides. 
Besides, there is nothing to prevent us conceiving, in regard 
to compounds so volatile and penetrating, that their action 
should extend, on parting from the centre where they are set 
free, over considerable thicknesses, such as those of the cry- 
stalline rocks of the Alps or Brazil. Sometimes the substi- 
tution of the silicates thus formed has been only partial, as 
