DR. JOHN S. FLETT: PETROGRAPHICAL NOTES ON THE PRODUCTS OF 
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and hornblende have more or less completely vanished. In their place we have 
aggregates of dark brown biotite, pale green augite, and hypersthene in small grains 
which may have parallel orientation, so that they build up skeleton crystals the 
interstices of which are filled with quartz and felspar. These aggregates do not form 
good pseudomorphs, and the minerals they represent cannot be identified by their 
outlines. They are strikingly different, however, from the idiomorpbic plagioclase 
crystals. The process of alteration can be seen in certain large crystals of brown 
hornblende which are rather common in these slides. The amphibole is reduced to a 
series of irregular patches (Plate 26, fig. 3), which can be recognised as having belonged 
to one crystal from their parallel orientation and simultaneous extinctions. The 
interspaces are filled up by granules of hypersthene and scales of dark brown biotite. 
These are the only rocks from St. Vincent with the exception of the norites and 
andesite-hornfelses in which biotite has been observed. In these secondary aggregates 
pale green augite occurs also, so similar to the hypersthene in colour and mode of 
growth that it can hardly be distinguished except by its optically positive character 
and its lack of pleochroism. The hypersthene has a large apparent axial angle, 
though it is pale green, and is always optically negative. Very little iron oxide 
occurs in these aggregates. 
The groundmass consists of quartz, felspars, hypersthene, tridymite and an isotropic 
material. The quartz may be in irregular grains ; very often, however, it is in perfectly 
formed double pyramids which, when the matrix is dissolved away, are beautifully 
sharp. It contains few enclosures, mostly tridymite. The felspars are of many 
kinds ; some are highly zonal and consist mainly of labradorite at their centres with 
margins of andesine; there are also irregular plates of untwinned alkali felspar which 
are covered with scales of tridymite. Hypersthene appears in the groundmass as 
grains and as perfect minute prisms of the usual shape. Iron oxides are practically 
absent. An isotropic material surrounds and separates all the crystalline minerals of 
the groundmass in very thin colourless or yeliow films. It is dissolved by boiling 
with caustic potash for some time, and this suggests that it may be a form of colloid 
silica, but its refractive index is 1’484, but varies slightly, and no kind of opal is 
known which has so strong refringence. It has also very nearly the same index of 
refraction as the more acid varieties of andesitic glass which were examined by 
Professor Lacroix* in Martinique. 
It is clear that these rocks have undergone alteration. Their pale colour shows 
that they contain but little iron; the great abundance of quartz in many of them 
leads us to suspect that there has been an introduction of silica. The transformation 
of the hornblende might be ascribed to fusion, or to contact alteration. That the 
changes have taken place at a high temperature is clear from the occurrence of veins of 
pyroxene traversing the groundmass, and filling cracks in porphyritic felspar. This 
shows that after the rock had consolidated there was a formation of augite. They 
* Op. tit. p. 511. 
