PHONOLITIC TRACHYTES. 
115 
Closely related to the preceding, but of a more definite trachytic type, are other 
ash-gray rocks, which show under the microscope small, sharply defined rectangular 
phenocrysts of anorthoclase in a ground-mass consisting of a trachytic mesh of felspar- 
laths, with interstitial dull-green segirine-augite and a little magnetite ( see Plate VIII, 
Fig. 4). The felspar-phenocrysts show the minute twin-striations characteristic of 
anorthoclase, and similar striations can also be detected in some of the larger felspar- 
laths of the base. The segirine-augite is in small, prismatic crystals, and shows 
pleochroism from brownish yellow to dull-green. 
To this type belong most of the ash-gray or yellow (when weathered) trachytic 
rocks from Cape Crozier (248, 224, 251, 243, etc.), as -well as specimens from 
Inaccessible Island (802, 803, 807), Brown Island (598, 600), and Black Island (610). 
Of almost precisely similar character to the rock (248) from Mount Terror 
(Plate VIII, Fig. 4) are trachytic rocks (55) obtained from the dredge off Cape 
Wadworth, Coulman Island. 
Of extremely salic character is the phonolitic trachyte (607) from the middle of the 
crater of Brown Island. It is a white, friable rock, resembling a domite. It consists 
of a fine-grained aggregate of small felspars (mostly short rectangular, but some lath- 
shaped) without flow-structure, in the interstices of which are distributed a few 
magnetite- grains and, very sparingly, needles of dull-green segirine-augite ; a few grains 
of segirine and small crystals of sphene are also present. The felspars have a refraction 
near that of Canada balsam, and are doubtless mainly anorthoclase. With a high 
power, under the microscope, are seen numerous minute and very thin hexagonal and 
square sections of an undetermined mineral, which is isotropic or only very feebly 
doubly-refracting, and has a refraction markedly less than that of Canada balsam. 
That nepheline is present in the base, although it cannot be definitely distinguished, 
is indicated by the chemical analysis of the rock, the result of which is as follows 
under I* : — 
I. 
Ia. 
II. 
(Brown Island.) 
Mol. ratio. 
(Mont Miaune. 
Si0 2 = 58-G4 
•977 
CO 
— 
TiO, = 0-28 
•003 
trace 
Al.A = 22-55 
•221 
19-G6 
Fe 2 0 3 = 0-97 
•00G 
3-43 
FeC) = 0-99 
•014 
— 
MnO = trace 
— 
trace 
CaO = 1-43 
•023 
1-53 
MgO = 0-10 
•004 
0-31 
Na,0 = 9-87 
•159 
10-04 
ICO = 4-98 
■053 
4-71 
P 2 0 5 = trace 
H 2 0 at 110° = 0-081 
— 
— 
H 2 0 above 110° = 0 - 35) 
1-00 
S0 3 = 0-27 
100-30 
99-4G 
Q 2 
For discussion of the analyses, see p. 119. 
