T. H. Waller—A Lava from Montserrat. 291 
Austria on the low extinction angles of the dark brown hornblendes 
when thus examined, as compared with the paler and green varieties. 
A few cases of twinning occur, in one case the orthopinacoid is 
evidently the composition plane according to the usual law. 
The crystals contain inclosed in them grains of the rhombic 
pyroxene, to be next mentioned, felspar, opaque grains with no 
apparent crystalline form, and some little rounded inclosures, which, 
however, do not appear to be portions of the glassy ground-mass, 
such as are contained in the felspars, since they have no bubble and 
no microliths. 
I have detected a few crystals of a green mineral showing no 
pleochroism and possessing oblique extinction at considerable angles 
with the length of the crystals. It has rather the look of augite, but 
I have not been able to come to any certain conclusion about it. 
A truly rhombic pyroxene is however certainly present in con- 
siderable quantity, partly in well-formed crystals, partly in grains 
of irregular outline, which however frequently show cleavages. 
The crystalline sections across the prism show rectangular forms with 
the prismatic faces only very slightly developed, only appearing in 
fact as slight truncations of the edges formed by the pinacoids. 
This it may be mentioned is exactly contrary to the habit of the 
augite in the lava from St. Lucia, mentioned above. In this the 
prismatic faces are most developed, and the pinacoids simply truncate 
their edges. The cleavage parallel to the prism is constant, that 
parallel to the brachypinacoid is frequent, while there are a few 
instances of the other pinacoidal cleavage. 
In these sections the colours of the two axes of elasticity a and 6 
are almost exactly the same, viz. a reddish-yellow, only differing in 
depth of tint as the object is rotated over the polarizing Nicol, one 
of the axes showing a slightly redder colour with a little more 
general absorption. 
In those sections, on the other hand, which show by their longer 
shape and the striation due to cleavage that they are more or less 
parallel to the vertical axis, the colour is a bright green when the 
length of the crystal (the vertical axis) is parallel to the principal 
section of the polarizer, and reddish yellow when the direction is at 
right angles to this. The extinction is always such as shows rhombic 
symmetry. This is most conclusively shown in the case of fragments 
picked out from the rock and examined both fixed by wax on the 
point of a needle so that they can be rotated, and also mounted in 
balsam. The difference in appearance between this pyroxene with 
its glassy transparent fracture and the hornblende with its brilliant 
opaque almost metallic lustre made the selection easy. 
I conclude, therefore, that the mineral is hypersthene. Allowing 
for its perfectly fresh unaltered condition, it is wonderfully similar to 
the mineral in the Cheviot porphyrite previously mentioned, with 
the possible exception of a somewhat more pronounced pleochroism. 
This, however, may be due to the fact that my slides of the Englsh 
rock are too thin to show the phenomenon in question to perfection. 
The similarity extends to the inclusions, which are ill defined, 
