THE ERUPTIONS OF MAY, 1902, AT THE SOUFRIERE IN ST. VINCENT. 
309 
which rose from the crater of Pelee in the latter part of 1902 proves that the magma, 
once solid, is not highly explosive. 
The Bombs. 
The slaggy black bombs, which were abundant especially on the surface of the 
new ash deposits in the valleys of the Wallibu and the Rabaka Dry River, all consist 
of hyalopilitic hypersthene andesite usually containing accessory olivine and sometimes 
hornblende. They are highly vesicular throughout (Plate 26, fig. 4). A description of 
their general appearance has been given in the first part of this report. The 
felspars are the most conspicuous minerals in the hand specimens, some of the larger 
phenocrysts being more than a third of an inch in length. The pyroxene and olivine 
formed smaller crystals easily visible without the aid of a lens. The olivine is 
yellow-green in colour; the matrix is dark grey to black. 
The abundant porphyritic felspars are always much zoned, the centres consisting of 
irregular highly corroded remnants of very basic plagioclase. Around these are 
successive deposits of felspar varying in composition, more basic and more acid bands 
often alternating repeatedly. The margins are more acid and more uniform in 
composition, and are often bounded by good crystalline faces similar to those found in 
the felspars of the dusts. Albite and Carlsbad twinning are almost universally 
present, and pericline twinning very frequently. Baveno twins occur, but are rare. 
Glass cavities with bubbles abound in the felspars, and often have a zonal 
distribution. 
Owing to their perfect freshness these felspars are easily determined by modern 
optical methods ; their specific gravities are not to be relied on because of the complex 
zoning and the glass enclosures, but fragments from the interior of a large phenocryst 
sank in a liquid at 2 - 745 (temperature, 2° C.), indicating 85 per cent, of anorthite in 
the felspar. The method of determination adopted wherever possible was the 
measurement of conjugate angles of extinction in Carlsbad-albite twins cut perpen¬ 
dicular to the zone of symmetry of albite twinning. The stereograms of Rosenbusch* 
were used, which differ somewhat in the basic end of the plagioclase series from those 
of Michel Levy. Consequently the proportions of anorthite in the felspar are 
slightly lower than those given by Professor Lacroix, who employed Michel Levy’s 
diagrams, but the results accord closely if we allow for this. The values given by 
Professor Becke in his recent papers on the optical orientation of the plagioclases 
were also used wherever possible. Determinations by the position of equal illumina¬ 
tion in zoned sections, perpendicular to the plane of symmetry of albite twins, proved 
useful as confirmatory evidence, though not so valuable as conjugate extinctions. 
Sections perpendicular to the bisectrices were not frequently made use of, as often 
* Rosenbusch, H., “ Mikroskopische Physiographie,” Band I., Heft 2 (1905). Michel Levy, A., “ Etude 
sur la Determination des Feldspaths ” (1894, 1896, 1904). Becke, F., “Die optischen Eigenschaften der 
Plagioklase,” ‘Tschermak’s Min.-Pet. Mittheil.,’ vol. xxv., p. 1 (1906). 
