292 T. H. Wailer—A Lava from Montserrat. 
rounded, apparently crystalline masses, and a few needles of some 
rather opaque substance in addition to the opaque grains mentioned 
as occurring in the hornblende. 
The felspar mostly presents the usual twinning of the plagioclases, 
but some of the crystals are simple. Seeing that in 14 measure- 
ments of the angle between the extinctions in a zone approximately 
perpendicular to the plane of twinning, as shown by the equality of — 
the extinction angles on both sides of the plane, this angle varied 
between 334° and 78°, mostly much more nearly the latter, it 1s pretty 
sure that the felspar is either labradorite or anorthite. Indeed, I 
believe that both are present, for some of the grains show the brilliant 
hard contrasts in polarized light which Fouqué and Levy speak of 
as characteristic of the more basic felspar, and in applying Szabo’s 
flame reactions to fragments picked out of the rock some were less 
fusible than others. I may say that the flame reactions point to 
a lime felspar containing about 3 per cent. of soda and 4 per cent. of 
potash ; that is, the potash colouration seemed very slightly more, and 
that due to soda considerably more than in the case of an anorthite 
containing 0°88 of the former and 1:22 of the latter. 
The probability of the presence of two felspars is increased by the 
fact that in one crystal which gave extinctions of 24° and 24° in two 
twin lamelle, there were portions which extinguished 41° from the 
trace of the twinning plane. The central part did so, then there was 
a zone extinguishing at 24°, then another at 41°, and then an outer 
one at 24°. The various zones were not very sharply divided from 
each other, but seemed to pass gradually from one to the other. 
The zonal structure is quite common, and in the case of many of 
the crystals no position can be found in which the whole area is dark 
between crossed prisms. 
Compound twinning is present, in some cases two systems of 
lamellee cross nearly at right angles, while in others two individuals 
showing the ordinary albite twinning are compounded after the 
Carlsbad system. | 
The inclusions consist partly of microliths, and partly of the 
glassy ground-mass of the rock. The microliths are in many cases 
arranged approximately parallel to the faces of the crystals, and are 
crowded together in some bands, while they are nearly or altogether — 
absent from others. The “glass cavities” are very numerous, and 
are wonderfully beautiful. They are of all shapes and sizes, a few 
being “negative crystals,” the others quite irregular. Crowded into 
zones when minute, more evenly distributed when large, in some 
places they form by far the greater part of the crystal section, the 
actual felspar substance being only the connecting matter quite sub- 
ordinate in quantity. Even when quite small they contain a cavity, 
mostly spherical, though sometimes elliptical in outline or somewhat 
irregular. In one case a little drop of glass has adhered to a micro- 
lith, like a drop of treacle to a glass rod, but it nevertheless contains 
its cavity. The glass of the inclusions is apparently slightly brown, 
and is filled with dark slender acicular microliths, in many cases 
