THE MEDITERRANEAN NATURALIST 
and Phase IV., periods 1 and 3, of Monte S'pmma. 
The above mentioned authors found the limit of 
temperature rather wide in which this mineral 
crystallized, which accounts for its inclusion in 
others that separate at rather higher temperatures. 
The pyroxenic glass seems to be the principal me- 
dium in which the other silicates and oxides are 
dissolved in basic rocks, whereas an acid felspathic 
glass seems to perform the same function in acid 
ones. 
We may regard the magma from which results 
an igneous rock as a variable mixture of acids and 
bases, as pointed out by Abich. Now, as consoli- 
dation takes place, great excesses of either, espe- 
cially the feebler ones, such as magnetite, are com- 
pelled to separate; and as the rock completes its 
crystallization, the excesses of either form the last 
crystals, unless the rock suddenly cools before all 
the vitreous matter has been converted into for- 
med material. Thus, in the acid rocks we have 
quartz, and in the basic ones magnetite, being the 
last formed minerals, although the two most infu- 
sible of rock-forming minerals, which alone is suf- 
ficient to demonstrate that fusion-point has little 
or nothing to do with the order of separation of 
the minerals. We should therefore be more 
justified in determining whether a rock should be 
regarded as acid or basic by its microscopical 
structure, than by adopting 60 per cent, of silica 
as rigidly dividing the two, since the different 
bases vary much in alkalinity, and combining pro- 
portions, and a magma containing 60 per cent, of 
silica, might give an acid or an alkaline reaction, 
according to the quantities of different bases it 
contained. 
Limit of space prevent further consideration of 
the different mineral species which go to make up 
igneous rocks; the above, being most common, are 
sufficient to indicate the line of argument followed 
out Before, however, quitting the subject, there 
is one more point worthy of our consideration in 
relation to the separation of mineral species from 
a solvent. Different species have been easily ob- 
tained from fusion of their components in a saline 
substance, such as a chloride or sulphate. Thus, 
for instance, M. Lechartier (1) obtained pyroxene 
in crystals, a centimeter long, by fusion for a 
couple of hours in calcium chloride, or sodium 
sulphate. In the same way wollastonite, apatite, 
(1) and many other minerals have been obtained 
by E. Belmen as very perfect crystals from solution 
in fused chlorides, and other salts, such as vana- 
dates. These facts go to confirm what has been 
said about the solution of the more infusible silica- 
tes in the more fusible ones, and at the same time 
may account for the occuirence of some minerals 
that are eruptive, or post-eruptive, in time of their 
formation. The large amount of sulphates, but 
especially chlorides, that are vaporized during an 
eruption is hardly credible until a few facts con- 
vince us that such is the case. I have seen fuma- 
role chimneys having in a short time their whole 
interior glazed by a mixture of chlorides, one to 
three centimeters thick, and from the intense heat 
as transparent as an ice covering, which was, 
without doubt, the result of sublimation, and not 
decomposition, as the rocks upon which it was 
deposited were quite unaltered. Another proof of 
the large amount of saline substances ejected by a 
volcano is the quantity met with in the falling 
ashes during a lava eruption. The outburst in 
1872 produced an ash asserted by Prof. Palmieri 
(2) to be poorer in soluble constitutents that any 
other since 1855, yet it contained from 4 to 9 per 
cent, of saline matter, chiefly sodic chloride. As 
this eruption was lateral, the principal part of the 
ash was derived from the crater edges and chimney 
walls, which would tend to lower the amount of 
soluble portion. 
It was observed in the eruption of 1855 (3) that 
the alkaline chlorides were only evolved sometime 
after the lava had been cooling — that is to say, 
saline crusts only formed around the fumaroles at 
a late date; and I have noticed the same thing. 
Scacchi supposed that it may be a spontaneous 
rise in the temperature in the lava in cooling, 
similar to that developed in phosphate of lead, 
nitrate of copper, or argentic (4) iodide when pas- 
(1 ) L. Bourgeois, Encycl. Chim., vol. ii., Ier 
Append ice. lie-prod. Artif des Roches, p. 10. 
(2) Annali del Reale Osserv. Meteor. Vesuviano 
187J h p. 73. 
(3) Guarini, Palmieri , Scacchi. Mem. s. Incend. 
Vesuviano del mese di Maggio, 1855, &c., pp. 141, 
I.43, and IV). 
(4) G. F. Rod well , Phil. Trans. R. S., Part Hi., 
p. 1134. 
(1) Comptcs rendvs, 1808, vol . Ixvii., p. 41. 
