ee 
ve 
DESCRIPTION OF ROCKS AND Ae eRe 37 
‘It is a dense rock, black in colour, only a few specimens repre- 
senting minute phenocrystals of plagioclase. The densest rock 
shows irregularly dispersed smooth valled cavities, empty on the 
air exposed rock surface, but containing a whole individual of cal- 
cite only a few millimetres beneath the surface; and only one cen- 
timetre or two deeper these calcite individuals are replaced by black 
sphaerules, which easily detach from the smooth walls of the cavity. 
Typical vesicular surface rocks are not represented. 
The mineral composition is a simple one. The densest rock is 
rich in yellowish-brown glass, but the grained ones also contain 
important quantities of it. 
The grained rock consist of albite-twinned microphenocrystals of 
plagioclase corresponding to labradorite, with a feeble zonar struc- 
ture; the measurements gave an anorthite content lying between 
the extremes of 43°/o and 70°/o, the greater part indicating narro- 
wer limits: 56% and 60% An. The needle-formed microlithes of 
the groundmass generation were determined as corresponding to 
Ab,,An,,, the limits being an anorthite content of 43 and 55 %o. 
The contours of the microphenocrystals are sharp whilst the outlines 
of the microlithes often are rough. These feldspars are dispersed in 
the brownish glass without visible order or fluctuation arrangement. 
— A yellowish augite appears also in two generations, the micro- 
phenocrystals showing distinct prismatic forms corresponding to 
(100) (010) (110), terminations (001) and (111), and twins after (100), 
or radiating groups with centrifugal growth, or irregular intergrowths, 
in some degree influencing the feldspar growth. The optical pro- 
perties as follows: 
City == AD 
7—B = 0.0226] 
2 V7 = 43°, with axial plane in (010); 
axial dispersion around y: A—e =v 
B-—o>vu 
