Dr. H. J. Johnston-Lavis—Trachyte at Naples. 75 
or greenish brown, and comparatively slightly pleochroic. The 
smaller crystals are also very irregular in shape, and range down in 
size to large microliths. Even when a comparatively well-formed 
rod is examined, no extinction can be obtained, the whole under- 
going aggregate polarization, as if some molecular change had taken 
place, or as if each individual were constructed of an association of 
minute grains. By changes that have occurred in much larger 
crystals of extraneous enclosed masses, it would appear that these 
microliths and crystals really consist of a mixture of pyroxene and 
amphibole. Another fact is that the greatest darkening is obtained 
at two points in rotating through 90° between crossed Nicols 
corresponding with the extinction of these two mineral species. 
The other variety of amphibole is certainly more recent in formation, 
but seems to have undergone the same molecular change, for it is 
almost impossible to get any definite extinction. The crystals are 
well formed, and associated with cavities, and no doubt are the same 
as those that project as long rods from the sides of the vesicles of this 
rock. In colour, they are very dark bluish-green, faintly pleochroic, 
but their opacity prevents a just estimation of such. 
The ground-mass consists principally of sanidine microliths of 
large size in fact, so large that in many cases Carlsbad twinning can 
be made out. Interspersed amongst these felspar microliths, and 
those of amphibole already mentioned, are numerous grains of 
magnetite, which apparently have crystallized mostly as octahedra, 
their form being partly obliterated by a rusty iron stain surrounding 
them. Rarely, highly refracting colourless grains occur of uncer- 
tain character, and within some felspars long four-sided ortho- 
rhombic? prisms with domes, of a fibrous-looking mineral, occur, 
which in some sections looks much like a triclinic felspar in struc- 
ture. Some attempts were made to isolate one of these, but unsuc- 
cessfully, and, on account of their rarity, none other suitable examples 
could be met with. 
Some parts of this mass of trachyte are vesicular, though the 
cavities are not very close together. There are also a certain number 
of fissures formed before the rock had quite consolidated. These 
vesicles and fissure-spaces are lined by a number of minerals which 
constitute the beauty and striking characteristic of this trachyte. 
Projecting from the walls of these spaces, long pseudo-hexagonal 
prisms, sometimes solid, at others tubular, occur; in the former case 
they are usually colourless, and as limpid as cut glass. In size they 
range up to and beyond one centimetre long, and from one to four 
millimetres broad. They may be traced by gradations to fine well- 
formed dodecahedra of sodalite, which are somewhat white or milky 
in colour. Associated with these sodalites are numerous rods of 
black amphibole, thin and long, usually about a millimetre broad 
and from a half to one centimetre and a half in length. Much rarer 
and irregular hollow-faced crystals of a black colour, with usually a 
rough submetallic lustre, which is commonly steel-black, but may 
be of dark bronze colour; most of these crystals are imperfect, and 
frequently arranged in chains. As far as I could make out, they 
