OF MONTE SOMMA AND VESUVIUS. 76 
irregular fissured interior mass. This straining may in part be due 
to glass cavities, which in some cases are very abundant in the 
mineral. ‘This vitreous material nearly resembles in colour that 
contained within the pyroxene crystals, and is very rich in gas 
vacuoles. Sometimes these glass enclosures are arranged in a 
stratified manner, and, seen edgewise under a low power, might 
lead to the supposition of triclinic characters in the felspar. One 
example showed a peculiar concentric arrangement. Within was a 
well-terminated sanidinic crystal, enveloped in a flecked dirty mass 
of the same mineral, which had an irregular eroded surface; both 
the nucleus and its envelope polarized and extinguished alike. 
These two were again enclosed in a clear crystal of the same mineral 
polarizing in different colours and extinguishing at a different angle 
from the contents. Other crystals have been partly dissolved and 
again covered with numerous irregularly arranged crystalline facets. 
This mineral sometimes supports on its surface little plates of 
biotite. 
Scattered rather sparsely through the matrix are microliths of 
pyroxene. ‘They are well formed, but may possess ragged extremi- 
ties to the prism, and, like the larger examples of the same mineral, 
they are twinned. Nearly all contain minute elongated cavities. 
In size they are very variable, ranging from almost invisibility to 
0:06 millimetre in length and 0:002 millimetre in breadth. 
There is to be seen rarely a bunch of sanidine microliths. 
The ground-mass is a slightly dusty brown glass containing a few 
very minute globulites, and broken up by the vesicles, which are 
often drawn out into long tubes that sweep round a larger ge ltlen 
space, so as to assume most varied forms. 
Remarks.—Any one who studies this pumice cannot but be ee 
with the peculiar features of it, which so plainly tell the tale of the 
prehistoric outburst that gave forth these volcanic products. 
The eruption seems to have commenced feebly, so that its earliest 
ejectamenta could at first not succeed in reaching beyond the enceinte 
of the crater, into which they must have fallen back. Such action 
would be favourable to the escape only of ashes at first. The 
igneous magma would be able to develope its crystalline structure, 
and the pumice fragments would thus obtain their rounded and worn 
surfaces. The thin intercalated ash-bands would point to small 
intermissions or diminutions of expulsive power, which in the middle 
subdivision must have been sufficiently strong to eject the magma 
in the form of pumice, but which could not have eroded the crater 
much, as proved by the absence of leucilitic lapilli. The forces 
seem to have gradually diminished, resulting in the ejection only of 
fine ash, which composes the dusty yellow upper subdivision. 
It is possible that the deposits above described do not represent 
all the products of the eruption, since if the north wind were blowing 
with great force the ejectamenta would be found chiefly on the south 
side of the mountain, where there are no existing sections to reveal 
these beds. 
