310 DR. JOHN S. FLETT: PETROGRAPHICAL NOTES ON THE PRODUCTS OF 
much time is required to find satisfactory ones. The optical sign in convergent light 
is easily observed and often valuable. The microlites of the groundmass were 
determined by their refractive index as measured against oils and Canada balsam, 
their maximum extinctions in sections from the zone 001,010 (longitudinal sections), 
and their extinctions in sections perpendicular to this zone. 
The most basic cores of the porphyritic felspars contain about 85 per cent, ot 
anorthite; a great part of the crystal yields extinctions almost identical with those 
given by Rosenbusch for bytownite (with 75 per cent, of anorthite). The outer 
margin is more acid than this, having usually about 50 per cent, anorthite, while the 
external borders may have as low as 40 per cent. A single crystal may thus have 
zones which vary from the composition of bytownite-anorthite to that of andesine. 
The groundmass felspars are usually long and narrow, often branched or hollow at 
their ends, and with few glass en closings. They belong to andesine, having usually 
40 to 35 per cent, of anorthite. 
The augite is pale green or greenish brown, and is free from zonal and hour-glass 
structures. Many of the crystals are idiomorphic, but others have irregular outlines, 
as if corroded. Repeated twinning on the orthopinacoid may occur. Transverse 
sections often show good outlines with the pinacoid faces as large or larger than the 
prisms. The extinction angle Z : c is 45 or 46 degrees, and the axial angle 2E is 
110 degrees. The hypersthene is usually idiomorphic, its prisms being four or five 
times as long as broad. It has eight-sided cross-sections, with the prism faces small, 
while the pinacoids are large. It is optically negative with axial angle 2Y over 
60 degrees and the usual pleochroism of this mineral in andesitic rocks :—Z = c green, 
Y = b reddish brown, X = a paler reddish brown. Parallel growths of augite and 
hypersthene occur, the augite being external. Both these minerals frequently contain 
enclosures of glass and magnetite. 
Olivine is not always present, but it occurs in more than one-half of the microscopic 
sections of these bombs. It is never very abundant, and may be regarded as an 
accessory mineral. In most cases its grains are rounded, though occasionally they 
present crystalline outlines. Evidently the mineral has suffered corrosion in the 
period which preceded the eruption, and this accounts for its rounded shapes and the 
frequency with which it is surrounded by borders of granular bypersthene. The 
formation of hypersthene in andesites of this sort by corrosion of olivine has been 
described by many writers, e.g., in the rocks of Martinique. Many of the crystals, 
however, have no such border, probably because the movements of the still liquid 
matrix, during the rise of the magma, swept away the hypersthene grains from 
around the olivine. The two minerals are never in regular parallel growth, and the 
hypersthene grains have no constant orientation with reference to one another. 
Agglomerations of hypersthene also occur which probably occupy the place of a former 
olivine crystal (Plate 26, figs. 1 and 2). It may be mentioned that another feature of 
these rocks, which is very constant, is the presence of glomero-porphyritic groups of 
