290 T. H. Waller—A Lava from Montserrat. 
IJ.—A Lava From Montserrat, West InpIzs. 
By Tuomas H. Water, Esq. 
[ the March and April Numbers of the GrozrocicaL Macazine, 
Mr. J. J. Harris Teall describes some beautiful rocks from the 
Cheviots, containing a rhombic pyroxene with well-marked pleo- 
chroism, and as there seems little doubt that this mineral, which we 
must consider hypersthene, has been hitherto overlooked in a good 
many rocks, another note of its occurrence may perhaps possess some 
interest. 
My friend Mr. Joseph Sturge, who visited the West Indies during 
the past winter, kindly brought me some specimens of lava from one or 
two localities. One of these, from St. Lucia, presents no feature of 
special interest. It consists of a triclinic felspar, augite and olivine 
imbedded, as crystals more or less fully developed, in a very opaque 
ground-mass, which, where it forms thin inclusions in the felspar, is 
seen to be a brownish-grey glass. The augite is very frequently 
twinned, and one crystal shows a pleochroism similar to that of hyper- 
sthene and extinguishes parallel to its cleavage striation; but I have 
not been able to detect any more instances of such properties. The 
piece I had was very small, and only served for the preparation 
of two slides; possibly a more extensive set would reveal further 
points of interest. 
The other lava from the island of Montserrat is a far more beautiful 
rock. It presents the appearance of a mass of very light-grey 
colour, so vesicular as almost to deserve the name of a pumice, con- 
taining obvious crystals of hornblende. The felspar crystals only 
show themselves by the reflections from their cleavage faces. The 
whole is so incoherent that the preparation of slides for micro- 
scopical examination proved an extremely difficult task, and the 
best that I have been able to make leave much to be desired in the 
matter of thinness. They are, however, sufficient to establish the 
following conclusions. 
The hornblende is in well-defined crystals, though some of these 
would appear to have been afterwards subjected to some agency 
which has rounded off their angles and edges, leaving their sections 
almost elliptical in outline. The crystals show the characteristic 
cleavages well developed, and are of a very dark colour, showing the 
following axis tints: a and ¢ brown and yellow, b a greenish-brown. 
But the absorption c=b><a is so strong that where the sections are 
moderately thick, they appear perfectly opaque. In one instance it 
is strong enough to enable the hornblende crystal to act as an 
“analyzer,” a little felspar running obliquely through the horn- 
blende, showing the usual play of colours when only the polarizer 
is employed. 
Fragments detached by cleavage showed an angle of extinction 
referred to the prism edge, averaging just over 10°, agreeing with 
F. Becke’s remarks in his paper in Tschermak’s Mittheilungen fir 
Mineralogie und Petrologie on the Gneiss formation of Western 
