284 
'THE MEDITERRANEAN NATURALIST 
the upper part of the chimney for a long time, and 
will have been loosing its heat in a very gradual 
manner, so that such minerals as leucite and pyro- 
xene at Vesuvius, or the latter mineral at Strom- 
boli, are able to gradually increase in size and 
perfection, which will proportionally diminish the 
crystallizability of the remaining vitreous matter. 
Prof. Samuel Haughton (1) has shown that the 
remaining paste consists of a very fusible basic 
glass with an approximate composition of 2R0,Si02 
containing much iron protoxide. On the other hand 
a large supply of lava brought up from below 
with considerable rapidity has little time for the 
growth of individual crystals, but the whole mass 
undergoes a microcrystalline change until no, or 
very little, vitreous matter remains to feed the far- 
ther increase of individual crystals. We have a 
parallel in such a case as the following; — If we 
make, a solution of some salt very soluble in boiling 
water, but very slightly so in cold, and we cause 
such a solution to cool moderately quickly, the salt 
will separate itself in a granular crystalline state; 
but if such cooling be made to take place gradual- 
ly during many days, very fine, perfect, and large 
individuals will replace the granular types. Now, 
when, a microcrystalization takes place, it will so 
separate the remaining vitreous material that even 
under the microscope little will be discernible, so 
that it is very difficult to detect it or appreciate its 
a,mount. But where suitable conditions favour the 
growth of large crystals in a similar magma, the 
vitreous matter that remains will be more concen- 
trated, and therefore more apparent both to the 
naked eye and under high magnifying power. 
The histological character of any cooling magma, 
with regard to its mineral components, is a ques- 
tion of profound interest, which, up to the epoch 
of the attempts of artificial reproduction of diffe- 
rent types, aided by microscopical research, remain- 
ed a very obscure subject. When we have to deal 
with the fused components of any single mineral 
in a pure state the researches of Messrs. Fouguh 
and Levy demonstrated that, so far as laboratory 
experiments go, the critical point of crystallization 
is near that of the fusing-point of a mineral. We 
1 “ Report on the Chemical , Mineralogical , and 
Microscopical characters of the Lavas of Vesuvius 
from 1631 to 1868V Trans. Roy. It . Acad., vol.xxvi., 
p. lift. 
should, therefore expect that in a leucitielaya, the 
leucite would be first separated as crystals, to be 
followed by felspars, and lastly by pyroxene. It is a 
J well known fa ct that some of these crystallize si- 
I multaneously. This is most strikingly illustrated by 
a coarse leucitic lava exposed near Orchi, on the 
volcano of Roccamonfina, where leucites. some two 
or three centimeters in diameter, eucl . ;e man y and 
perfect crystals of sanidine and pyroxene, whi.-h 
in some cases, are entirely enveloped, or protrude 
a short distance from their surface. One might, 
with such a series of contradictions, feel inclined 
to give up further experiments in the laboratory. 
Before, however, let us compare what has been done 
by the chemist, and see if it is borne out 1 >y rocks as 
presented to our observation by nature. We will 
commence byrecalling the interesting researches of 
Sir James Hall (1) who noticed that I such igne- 
ous rocks as whinstones and basalts as were fused 
and cooled quickly, a glass resulted; but by keep- 
ing them near fusion-point (-•. uit of modern' 
French authors), or allowing rhern to cool slowly 
a crystalline structure resulted. These experiments 
were followed up by Gregory Wat, (2) who went a 
step farther, and demonstrated that the sp. _.r. in- 
creased in proportion to the prolongation of 
the time of cooling. 
The absence of microscopical research prevented 
any important inferences from being drawn from 
these early experiments, and it was not till the 
investigations of DaubiAe, Hautefeuille. Freidtl 
Sarasin Fouque, Michel Levy, and others that 
much advance was made. These authors (3) found by 
remit, more or less prolonged, the following mine- 
rals might be obtained from their fused chemical: 
components Peridote, pyroxene, nepheline, leucite, 
triclinic felspars, mellilite, gehlenite. and sphene; 
whilst from mixtures not corresponding to the 
mineral obtained, the following were prepared:— 
Tridymite, oxides of iron, and perovskite. Many of 
1 "Experiments on Whinstorus and Lava, 1188;' 
and also Trans. Roy. Coe. Edinh. 1805, rot. v., pp. 
8 and 56. 
2" Observations on Basalt and the Transition fern 
the Vitreous to the strong Texture which • curs in die 
gradual Refrigeration of the Melted Basalt , irit/i 
some Geological Remen ksL-Phil Trans., 1 PC ),p.27‘J 
3 Encycl. Chimique, tome if.,, Metalloids. Lr 
Appendice. Reproduction J rt if cal des min raux e 
rockes. L. Bourgsois, p. 10, 
