318 
DR, JOHN S. FLETT: PETROGRAPHICAL NOTES ON THE PRODUCTS OF 
a porphyritic appearance ; others have finer grained bands, which may possibly be 
veins. We obtained also some blocks rich in long bladed crystals of hornblende (over 
an inch in length). These are arranged parallel to one another, giving the rock a 
coarsely gneissose character; or it might be compared to a comby vein with prismatic 
crystals perpendicular to the boundary walls. There is little olivine in these 
specimens. Others have a drusy structure with small cavities lined by the termina¬ 
tions of idiomorphic crystals. None of the masses was of large size ; specimens over 
a foot in diameter were scarce. On the other hand, many of the fragments which 
lay near one another on the ashes were very much alike, as if they were parts of a 
large block which had been shattered on striking the ground. The great variety in 
the appearance of these blocks indicates that if they come from a single rock mass 
situated beneath the volcano, that mass must be very heterogeneous. These blocks 
were all exceedingly friable, and fell to powder when struck with the hammer, so 
that thin chips could not be detached from them, a property which will be explained 
when their microscopic characters are described. 
In thin sections none of the minerals have definite crystalline form. The olivine 
is rounded and often much cracked. The augite is pale green and not dichroic. Its 
extinction angle Z : c is”45 degrees; the optic axial angle 112 degrees. Twinning on 
100, simple or repeated, is common. The hypersthene has the usual pleochroism, 
not very intense : its axial angle 2V is over 60 degrees. The felspar, which is the 
most abundant constituent, very seldom is zoned. Pericline and albite twinning are 
both frequent; Carlsbad twins are few. Conjugate extinctions in sections of Carlsbad- 
albite twins indicate a high percentage of anorthite (85 to 90 per cent.). In convergent 
light the sections are negative. The specific gravity of the powdered felspar is on an 
average 2‘75. The refractive index and double refraction are both very high. The 
hornblende is brownish green, fairly deep in colour, and is often in parallel growth 
with the augite, the two minerals having their prism axes and zones of symmetry 
parallel. Sometimes the amphibole surrounds the pyroxene; at other times they are 
intergrown. The pleochroism of the amphibole is X yellow, Y yellow green, Z darker 
brownish green. Z : c about 13 degrees in sections showing vertical emergence of the 
optic normal. Y = b. 
All the minerals contain small glass cavities with a fixed bubble; they may also 
show large enclosures of brown glass. Some of the cavities seem to be empty. Fluid 
enclosures with mobile bubbles, though they might be expected to occur, were not 
seen. None of the minerals, though the rocks have plutonic structure, have the dark 
platy enclosures which produce schiller. As a rule there is no definite zonal or 
crystallographic arrangement of the glass cavities. 
In structure these rocks resemble gabbros and troctolites (Plate 27, figs. 1 and 2). 
The felspar has partly crystallised after the augite, and may be enclosed in it, but is 
never enveloped by olivine, which has probably been the earliest of the silicates. 
The rocks consist generally of nearly equidimensional grains, fitting together in a 
