310 
THE MEDITERRANEAN NATTRAEIST 
basic pumices to find sanidine (1) crystals eroded, 
enclosed in others, which in turn may exhibit 
eroded surfaces, and again be enclosed in a third 
crystalline shell with well-defined facets. The 
orientation of each crystal being different from 
that which it coats, or is covered by. It is evident, 
therefore, that this mineral must have undergone 
a series of vicissitudes which must have taken a 
far longer time than was occupied in the eruption 
and cooling of this product of an explosive erup- 
tion, and must have required more quietness than 
could occur in the expansion and ejection of pumice. 
This latter example I take to be an important 
argument in favour of the hydro-thermal, or plu- 
tonic, formation of orthor-clastic felspar in a 
magma cooling under great pressure. Another 
fact also of deep interest is the very extensive 
replacement of sanidine in the Yesuvian pumices 
of Phases III. and IV., by leucite in those of Phew 
VII. and the lavas, as these are the two principal 
competitors for the potash. If the granite and 
syenites of the Val di Fassa, and the latter of Skye 
described by Scrope aud Geikie respectively, are 
really subaerial expansions, which I doubt, we 
must suppose them to have been nearly completely 
crystallized before eruption. Porphyries, no doubt, 
are erupted granites, which had undergone much 
crystallization before their extrusion. Even in the 
most vitreous rocks, such as the obsidian and 
obsidian pumice of Lipari, where the latter, al- 
though, as a whole, a highly vitreous mass, contains, 
large crystals of sanidine scattered through its 
mass' 
Triclinic felspars. — Other felspars, such as labra- 
dorite, were produced by a recuit of some days : 
but large crystals, such as are met with in the 
Etna lavas, do not so far seem to have been obtain- 
ed. Such a result is easily explicable, when we 
are informed that to produce a microlith some days 
are require, whereas we know that even after the 
expulsion of the lavas of Etna many months or 
years are requisite for their cooling, so that recuit 
maybe prolonged far beyond the limits within 
which we can experiment. If a large stream of lava, 
such as issued from Etna in 1669, be examined, 
it will be found that even that which was cooled 
(1 ) II.J.J.L. “Geolog// of Mt. Somma and 
Vesuvius,” dec. Q.J.G.S., Jan., 1884, P- 71. 
Scion. yProc,, IUI.S. Vul. V. PT. HI. 
immediate! . contains crystals of la bradorite, which 
indicate the piutonic origin of that mineral. < r that 
the magma had been undergoing a pr ior teed 
recent in the npj me chimney. Speci- 
mens taken from the centre of some of the thicker 
parts of the stream far from its source, and which 
must have been long in cooling, we find the crys- 
tals of that felspar therei . . . 
greater dimensions, thereby indicating that under 
favourable circumstances this mineral -y under- 
go further growth after ext rush .. > i the lava. A 
similar occurrence I have noticed at Vesuvius. At 
Cisterna is a gigantic lava stream that is known 
to be more than half a kilometer broad, and its 
depth beneath is is quarried 
to a depth of twenty meters, at a distance t f more 
than ten kilometers from the original i-r.ip:iv ; u xis 
of the m >untain. Now, of all the 1 ivas . . 
Somma this is the most extremely crystalline one. 
all its constituents being cf very large size, and 
practically all the amorphous ; aste has passed into 
a crystalline condition. So far as is known, this 
is the greatest outpour that ever ■ meurred from, 
this mountain ; and, no doubt, in c >qs prance of 
its enormous thickness, being unrivalled ’ y any 
modern stream, a very long time must have b en 
occupied in its cooling, conditions highly fav oura- 
ble for the production of a coarse structure. When 
small streams dribble from the crater after pro- 
longed Strom b s also is 
often very coarse, as the part of the lava column 
iu the chimney has been gradually loosing its heat. 
Anyone who visits Monte Somma may have noticed 
that most of the lavas along its crest are coarse- 
grained, whereas most of those nea 
finegrained. The reason, as at present with Vesu- 
vius, is obvious. 
From the almost impossibility of artific: .iiy 
producing the felspars, Delesse, (1) Daubree. (•_) 
and Sorby, (3) assert that they must be the result 
of hydrothermal origin. Whether the actual jn\ - 
sence of water is necessary directly, or only as the 
(l) Fouque au<l Levy, Comptes rendus, 1878 , t. 
Ixxxvii., p. 700. 
(1) Bull. Sue. Geo/., 1857 and 185$. >vl xv., p. 
718, 7 57, 700. 
(I) Rapport sur h s prog 's d /./ gt.Jotju experi- 
ment-ale, 1807, pp. 08 and 8 4 . 
(J J Brit. Assoc . Beports, 1880. 
