THE MEDITERRANEAN NATURALIST 
Levy (l) obtained by igneous fusion and recuit 
without a dux. With the components of leucite 
alone it was impossible to obtain the mineral, and 
this could only be done by taking equivalent com- 
ponents of a mixture of that mineral and pyroxene. 
This is a most important fact that again helps to 
clear away the veil of mystery which overhangs 
the genesis of many silicates. 
Most substances can behbtained crystallized by 
one or more of four principal methods — from 
sublimation, by fusion, by evaporating a solution, 
and by cooling down asolvent. The necessary 
temperature is highestfor the first, less for the 
second, and very much the lowest for the third 
and fourth. Sulphur, to be obtained in crystals 
from fusion, require! a temperature of at least 
115° C., whereas by solution in carbon bisulphide 
we may obtain crystals far more perfect at the 
ordinary temperature of the air. 
We must, therefore, look upon leucite as dissol- 
ved in a medium which is liquid at a bright red 
heat, and only gives up this, as well as other mine- 
rals, by a lowering of temperature, in the same way 
that a mixed boiling saturated solution of salts of 
various solubilities separate out (far below their 
fusing point) as the solvent cools. Precipitation 
might also depend upon withdrawal from the 
mixture of one or more of its elements for the 
formation of a mineral that has already commenced 
to sepaiate. 
If we take a solution of mercuric biniodide in a 
solution of potassic iodide, and add some substance 
that will seize upon the iodine in the latter salt, 
such as argentic nitrate, we have an immediate 
precipitate of the mercuric biniodide proportional 
to the amount of potassic iodide broken up. Stop- 
pard gives the example of nitrate of potash dissol- 
ved in water, which is precipitated immediately if 
alcohol is added. (2) 
The fact, therefore, of leucite crystallizing far 
below its fusion-point proves the solution of that 
mineral in that glass or some other. 
This would explain the crystallization of the two 
minerals simultaneously, as at Eoccamonfina; for 
as the loweriug of temperature took place in the 
magma as the pyroxene crystallized out, the 
(1 ) Comptes rendus, 1878. t. Ixxxvii., p. 961. 
(2) C'/rso di Geologia , vol. Hi., p. 131. 
321 
remaining would become supersaturated with 
leucite, which would have to separate. We might 
possibly imitate this condition in freezing a satu- 
rated solution of a salt in water. It is also pos- 
sible that the leucite does not form until the potas- 
sic chloride in the magma has been broken up, and 
the HC1 has escaped in the vapour. 
In the formation of rocks we have a process of 
fractional exhaustion of the original amorphous 
medium, in which secondary combinations can 
hardly be conceived to take placeuntil some portion 
assumes definite crystalline form, the kind of 
which will depend upon the elements that enter 
into the composition of the mixture, and the train 
of conditions which that undergoes in passing from 
a higher to a lower temperature. 
Starting, for example, from an amorphous 
mass of fused silicates, we may suppose that 
condition 1 is favourable to the formation of mi- 
neral B. but as this separates, A can no longer 
remain in solution, so this also separates until 
the magma is deprived of as much of the ele- 
ments as these minerals A + B can take up, 
and the glass is then suitable for the growth 
of C which comes next, and in its turn may be 
followed by D, and so on. The resulting rock will 
be composed of the minerals A + B + C + D, 
Ac. Let us again start with the same magma, and 
suppose that condition 2 comes into play, which is 
favourable to the formation of A, which will sepa- 
rate, exhausting the magma to a point that is 
suitable to the formation of X, in preference to 
any other, which now carries the exhaustion on, 
till the magma approaches Y in composition which 
in turn continues the exhaustion, till the unformed 
material is suitable for the crystallization of D. We 
should thus obtain a rock containing the minerals 
A + X + Y + D, both of which would be identi- 
cal in ultimate chemical composition. Now, condi- 
tion 1 may have been favourable to rapid expansion, 
and eruption such as pumice results from whilst 
condition 2 we may take to represent the gentle 
outflow of lava. The realty of this somewhat rough 
illustration will be more apparent if we compare 
the vitreous pumices of Phases III. and VI. of 
Monte Somma, in which leucite is absent, and 
sanidine abundant, with the highly leucitic basalt 
lavas of the same volcano, in which sanidine at the 
most is a very an important element, remembering 
