THE ERUPTIONS OF MAY, 1902, AT THE SOUFRIERE IN ST. VINCENT. 
307 
absolutely identical origin. Those that were carried to some distance were emitted 
from the crater during the height of the paroxysm of the afternoon of May 7th. 
Those gathered near the Soufriere may have been mixed with materials emitted at a 
subsequent time, either during the first or second eruption (May 7th and 18th, 1902). 
Sand from the Soufriere contains, as might be expected, a larger number of fine lapilli; 
the Barbados dust consists mainly of single crystals, or fragments of crystals, with 
small grains of more or less vitreous character. But the same ingredients occur in all 
the dusts in very much the same proportions. There are slight differences in the 
relative abundance of the component minerals which can be traced very clearly in the 
chemical analyses of samples from different localities (see p. 311), but these differences 
are not great. 
Sir Daniel Morris has sent us samples of the Barbados dust-fall of May 7th 
and 8th, 1902, collected at different hours. These show that the material which fell 
during the later hours was finer grained and rather more pale coloured than that 
which fell earlier. It is to be expected that as the dust cloud was at a great height 
the coarser and heavier minerals would first subside. Fragments of broken felspar 
crystals and of glass are distinctly more common in the later part of the dust-fall and 
account for its paler colour. 
The most striking feature of these dusts is the large proportion of crystalline 
minerals they contain, and this led us to bring forward the hypothesis that the 
magma, “or at least the upper part of it which gave rise to the great black cloud, 
was to a large extent crystallised, contained comparatively little fluid matter, and 
was accordingly near the temperature of consolidation, or may even in part have solidified 
already” (Pteport, Part I., p. 523). This hypothesis received the most unexpected 
confirmation at a slightly later period when it was announced that the magma in 
Pelee was being extruded as a solid vertical column or obelisk which attained at one 
time a height of 900 feet. 
Of the minerals of these dusts the most abundant is plagioclase felspar. Its 
crystals are often broken, but the smaller may be perfect, especially when they have 
an adherent pellicle of glass. Optical and specific gravity tests, especially the 
extinctions of cleavage flakes, show that they range from labradorite to basic 
bytownite (An 50 per cent, to An 84 per cent.). They are full of glass enclosures, 
often of regular shape, with fixed bubbles. Zonal structure is very common in them ; 
in fact, they present similar characters to those of the phenocrysts in the bombs, to 
be described later. The commonest faces are 001, 010, 110, 110, 101, 201. The 
augite is pale brownish-green, very slightly pleochroic in thick grains. Good crystals 
are rare and show 100, 010, 110, 111 . The extinction angle Z : c is about 45 degrees. 
Hypersthene is always present in considerable amount and often its crystals are very 
perfect. Their faces are 100, 010, 110, and probably 111. Frequently the crystals 
are broken across, along planes of fracture perpendicular to the prism axis; they then 
show squarish octagonal outlines with the pinacoids larger than the prism faces. 
2 r 2 
