OBSERVATION HILL. 
13 
pure transparent felspar, while others have the mineral composition of gabbro and 
peridotite (see p. 107). Most of the coloured inclusions are quite angular, but a few 
are rounded ; the largest of them are about three inches long. 
On the south-east side of the crater a non-vesicular black olivine-basalt, 
approaching limburgite in character (326, see p. 105), forms a rugged hillock about 100 
feet high. The exposure of rock is quite 50 yards across, but the material is crumbling 
rapidly away and fresh rock is only obtainable near the summit. 
Observation Hill. At the extreme south-west point of Ross Island is Observation 
Hill, which is separated from its neighbour, Crater Hill, by a narrow col called 
The Gap. Observation Hill has very steep slopes which make an angle of 40° to the 
horizontal, and, almost meeting in a point, produce a strikingly pyramidal hill (Plate II). 
The south-west side slopes away more gradually and terminates in Cape Armitage. 
This prolongation appears to be due to the presence of a sheet of rock which is bedded 
horizontally. This rock (553) occupies but a small area, about 200 yards long and 
50 yards broad. The rock is a porphyritic olivine-basalt, almost black in colour and 
containing phenocrysts of green olivine up to one-eighth of an inch in diameter. 
Observation Hill appears to have been built up of successive Hows of trachytic lava 
which have welled up through one single outlet. These Hows are now to a great extent 
denuded ; but on the south-east side the remains of a sheet occupy the greater part 
of the hillside, and rest upon another similar sheet (412). The lower sheet spreads 
out and forms the flatter south-east side of the hill. 
On the north shoulder of the hill the trachytic lavas show a rather greater variety of 
texture, especially near the 400-feet contour. A dark-grey hornblende- trachyte (273 and 
281), with abundant lapilli-like inclusions (see p. 118), up to an inch in diameter, forms 
the shoulder on which a perched block of black vesicular basalt is prominent. The 
rock with these inclusions has a conspicuous platy structure, and the upturned edges of 
the slabs into which it weathers may be traced across the Gap to the base of Crater 
Hill. This platy rock is considerably contorted and its apparent strike is exceedingly 
variable, sometimes turning through more than two right angles in 50 yards. It is 
obviously older than, and unconformably overlain by, the yellow hornblende-trachytes 
(278, 279, 280) of the higher part of the hill. Higher up the hill occurs a dyke of 
grey hornblende-trachyte (277, see pp. 117 and 119). The dyke is not more than 10 feet 
broad, but is traceable 100 feet vertically up the hill. The top of the hill consists of a 
yellow trachyte ; locally it is streaked with grey ribbon-like bands (288) which follow the 
How-structure. The weathered surface of the rock is honeycombed, but as here the wind 
removes the snow immediately after its fall very little frost-action seems to take place. 
On the southern side of the summit the darker rock begins to preponderate, and at a 
point some 30 yards away from the top, and 50 feet below it, the yellow rock gives place 
to a dark-grey hornblende-trachyte (290). Below this rock comes another dark-grey 
hornblende- trachyte with spheroidal structure (655). The spheres which make up the 
mass of this exposure sometimes attain a diameter of over 2 feet and are visible over 
