THE MEDITERRANEAN NATURALIST 
a m icrocrystaJline mass, such as that of 1631 of 
Vesuvius, which is a very compact rock, yet gave 
forth enormous quantities of vapour as almost to 
resemble the explosive type of eruption. The lava 
of 1 80 S, which is rich in large leucite crystals and 
much interstitial glass, is a remarkable spongy 
structure, because its viscid nature prevented the 
escape of the few included bubbles of vapour, 
which, compared with others, was remarkably small 
in quantity in that eruption, affording compact 
types of lava surface. This escape of vapour may 
so separate the constituent minerals of a scoria 
surface as to leave it in a perfectly incoherent and 
pulverulent state. This I have seen in some of 
the trachytes of Ischia/and of the Solfatara (Monte 
Olibano ). 
Another fact is, that lava as it pours out, that 
portion which is nearer the surface will, in all 
probability, be the richest in water, and will pro- 
duce a stream thickly covered with scoria. But as 
the portion which comes from greater depths rises 
it will [have been exposed for less time to aquiferous 
rocks, and in consequence, containing less water, 
will produce a’smoother-surfaced steam. This was 
remarkably the case in the Vesuvian eruption of 
1855; 1 the first streams that issued were much 
rougher than those at a later date. 
The conditions under which the composition, of 
Igneous Rocks is modified. — One of the most vexed 
questions in geology is undoubtedly that of the 
variation in an igneom rock, and more especially 
with regard to its chemical than its mineral com- 
position. Space forbids here to enter fully into 
the theory of stratification of magmas, as repre- 
sented by Yon Richlhofen and others. No distinct 
division can be drawn between rocks derived from 
the most acid, and the ultrabasic magmas, show- 
ing that they can mix in all proportion. Then 
again, whatever be the silicates we may fuse 
together, none of them separate from each other, 
however long they may be kept in the fluid state. 
Thirdly, all magmas may be looked upon as origi- 
nally mixtures of fused Oxides, some basic and 
other acid, it is true; but in cither extreme types 
there is a certain amount of intermixture. We 
fin'! such substances as the fast, mineral oils, chlo- 
(1) Memoria. S. Incend,. Vesuvio, 1855 , <J. Gua- 
rini , L. Palmier i, and A. Scacchi. 
275 
roform separating from water, or mercury from 
either; but we must remember that these ineorn ■ 
patibles are built up of molecules, arranged on 
entirely different plans, which is not the case with 
the constituents of volcanic rocks. It may seem 
improbable, but I feel sure that time will show 
that the active cause of various rock composition, 
at any rate, to a certain extent, will be proved to 
result from some chemical changes brought about 
between an isolated portion of an original common 
magma and the neighbouring rocks. Also the 
infiltration of saline solutions may result in the 
bases of the contained salts, combining with the 
silica, and liberating the original acids. The 
facts that support such a theory are certainly few, 
but also those that can be urged against it are 
equally so, and in most cases can be answered. 
Thus, for instance, when great dykes, such as those 
that traverse the north of England for miles, change 
little their composition; and we hear, even at a 
most recent date, such an authority as Mr. Teall 
arguing against this theory; it does seem in a 
tottering state. We must, however, remember 
that in most cases we are only able to examine a 
dyke, over any large area, in its horizontal exten- 
sion; but what is really necessary would be to 
investigate such sheets of rock in their vertical 
extension. There are examples in various parts of 
the world where dykes that extend to some distance 
show alteration in composition as the rocks tra- 
versed change in character. 2 Von Buch and others 
have shown that in the Tyrol granite veins grad- 
ually pass into basalt ones, when traversing dolo- 
mitic limestone. The basalts of the Cyclopean 
Islands that are intrusive in a clay are most mark- 
edly altered where the dykes are thinnest. It has 
been shown that the great Whin-Sill has swallowed 
up beds roughly equal to its own thickness. On 
theoretical grounds we could easily understand air 
acid lava taking up limestone with its impurities, 
and becoming more basic, and thus reducing its 
temperature whilst it became more fluid. If this 
were the case we can understand that further action 
011 limestone would be limited by saturation of the 
(2) N. S. Shader. “ Propositions concerning the 
classification of Lavas considered, with 'reference to 
• the circumstances of their extrusion. ” Anniversary 
\ Memoirs of the Boston Society of Natural History , 
I 1880 , 
