324 
DR. JOHN S. FLETT: PETROC4R APIIICAL NOTES ON THE PRODUCTS OF 
equal proportions. The olivine shows resorption with formation of hypersthene ; 
the latter is often partly replaced by augite. The porphyritic structure of the 
andesites is repeated in this group. The large felspars are highly zonal, and have 
centres of bytownite (75 to 85 per cent, anorthite), while the margins are labradorite 
(about 50 per cent, anorthite). There is rarely much glass in the matrix of these 
rocks, and in this respect they show affinities to the basalts. Hypersthene basalts 
occur in small numbers among the ejected blocks of the Soufriere, and are found 
also at Chateaubelair, Cumberland, Buccament, and other portions of the island. 
According to Professor Bergeat the Somma of the Soufriere contains rocks of 
this type. 
The olivine basalts consist of plagioclase, augite, and olivine. Hypersthene is rare 
or absent and hornblende is not known to occur in them. Coarsely porphyritic and 
finely porphyritic types occur. Some carry large phenocrysts of felspar; others 
contain only porphyritic augite and olivine. Many of the latter group are very rich 
in olivine (Plate 27, fig. 6), which acquires a rusty brown colour as it weathers and 
ultimately passes into serpentine. The large felspar phenocrysts are much zoned, 
though not so markedly as those of the andesites. Their centres are similar to those 
of the felspars in the hypersthene basalts; the borders are labradorite and in the 
groundmass the small elongated microliths have labradorite centres with outer zones 
of andesine. Olivine may occur as small crystals of the second generation in the 
groundmass. The augite is less green and more brown in colour than in the 
andesites. Traces of a vitreous base are to be seen in some basalts near Cumberland, 
but this is not common. Tridymite may occur in cracks and cavities in the 
groundmass. 
Basalts have been described from Chateaubelair Point by Professor Lacroix. He 
compares them with the rock of Panders Island in Martinique. Professor Bergeat 
identified specimens from Buccament and the Soufriere. They occur also at 
Barroualee, Cumberland, Calliaqua, and Kingstown. Many of the largest lava flows 
appear to belong to this group. 
Two analyses (cited from Professor Lacroix) are given below, one of a hypersthene 
basalt, the other of an olivine basalt from St. Vincent. They show very well the 
decrease in silica and the alkalies, accompanied by an increase in lime and still greater 
increase in magnesia, which are to be expected from the mineralogical constitution of 
these rocks. Compared with the analyses of recent bombs, already given, they prove 
that the hyalopilitic hypersthene andesites recently erupted are the most acid lavas 
known to occur in the island. 
The petrography of St. Vincent, so far as is known, is less varied than that of 
Martinique, Grenada, and Dominica. Hornblende andesites, dacites, trachytes, and 
rhyolites apparently do not occur, and there is no evidence that plutonic rocks are 
anywhere exposed at the surface. The absence of the more acid types which are 
found in Martinique and Dominica is the most striking feature of the petrography. 
