THE TRACHYTIC ROCKS OF OBSERVATION HILL. 
117 
of this mineral are also included in the porphyritic felspars, and occur sparingly as 
phenocrysts. The anorthoclase-plienocrysts show no twin-striations, but have mottled 
extinctions ; the refraction is not appreciably higher than that of Canada balsam. 
The trachytic rocks of Observation Hill present a special type which approaches 
very closely to the “ tephritic trachyte ” of Forodada, Columbretes, described by 
Becke * and placed by Rosenbusck in the group of trachydolerites. 
The mineral which especially characterizes them is a reddish-brown basaltic horn- 
blende similar to that which is present in so many of the basalts. This mineral occurs 
in small prismatic crystals, only occasionally large enough to be detected by the 
naked eye. 
These trachytic rocks practically compose the whole of Observation Hill, near 
Winter Quarters. All the rocks described on p. 13 of the Report on the Field-geology, 
both the slabby rocks (281, 291), the spheroidal rock (655), the dyke-rock (277), and 
the yellow rocks (279, etc.), are very similar in microscopic characters, and only differ 
in the more or less glassy nature of the base or in the size and number of the horn- 
blende-needles. The yellow rocks from the top of the hill appear to be only weathered 
gray rocks. In specimens showing alternating bands of gray and yellow (see p. 13) 
these bands show under the microscope a precisely similar and continuous mesh of 
felspar-laths, and only differ in the yellow bands containing plentiful yellow grains of 
epidote in place of the minute magnetite-grains and small dull-green augites of the 
gray bands. These trachytes show no phenocrysts of felspar, augite or olivine. Under 
the microscope are seen small prismatic crystals of basaltic hornblende (or of black 
magnetite-pseudomorphs after it) in a trachytic mesh of minute felspar-laths, showing 
generally well-marked flow-structure, with thickly disseminated grains of magnetite 
and small needles of pale dull-green augite. 
In the dyke-rock (277) and in the dark rock (290) on the S. side of the hill the 
hornblende is less altered and in larger amount than in most of the other specimens, 
and the flow-structure of the lath-shaped felspars is less pronounced (see Plate VI II, 
Fig. 6). 
A black streaky rock with greasy lustre (264), half-way up the hill, is a glassy 
variety presenting some interesting features. Under the microscope it shows a few 
small prismatic hornblendes in a base consisting of a nearly colourless glass enclosing 
felspars, partly in sharply defined laths, but mainly in very thin hexagonal or nearly 
rectangular plates : small prismatic crystals, both of pale dull-green augite and of 
basaltic hornblende, and minute needle-like microlites of augite with a very little 
magnetite, are also present. In parts of the slide occur groups of minute colourless 
circular or roughly six-sided and eight-sided isotropic crystals (Fig. 64). Though they 
appear to have a high relief, the Becke-eflect shows that their refraction is less than 
that of Canada balsam, and they are here referred to leucite. Minute icositetrahedra 
* Becke, Tschermak’s Min. Petr. Mitth., 1896-7, Bd. xvi, p. 174. 
