50 
H. T. FERRAR. 
about 12 feet in diameter, and weather to a bright-chocolate colour. At its 
junction (668) with the sandstone it becomes finer in texture, and it has obviously 
altered the sandstone for a distance of two feet, at least, from the contact. At one 
spot a mass of sandstone lias been caught up in the sheet, and, near by, a pipe 
of dolerite 50 yards in diameter cuts vertically across the bedding-planes of the 
sandstone. The sheet extends along the side of the valley towards Finger Moun- 
tain and is interrupted by occasional small faults, the throw of which is always 
less than 100 feet. 
On the north side of 
and are capped by a dark 
or dyke of the dolerite 
cuts through the sand- 
stone and joins the mass 
above, and at y :i there 
are two sheets of dolerite 
separated by sandstone. 
Further east again, at 
C 7 , the cap of dolerite 
has been removed by de- 
nudation, and the lower 
sheet, which is reduced to 
under 100 feet in thick- 
ness, caps these isolated 
hills. Near the Inland 
Forts three dykes of the 
dolerite were examined, 
and a series of specimens 
(682-688) of dolerite and sandstone was collected along a line transverse to 
one of these dykes. These dykes are about 12 feet across and rise vertically 
through the sandstone to join the dolerite-sheets above. Where the dolerite meets 
the sandstone weathering has been accelerated, and the dykes lie in chimneys* 
which are sometimes 20 feet deep. One of these pipes is on the south end of West 
Groin, the other on the north end of East Groin and close to West Fort (C 9 ). 
Finger Mountain (B) (Fig. 22). 
This mountain is 7084 feet high, and is composed of alternate layers 
of sandstone and dolerite. The lowest rock visible at the base is Beacon 
The Inland Forts (Fig. 20). 
the Ferrar Glacier the hills are very uniform in height 
rock for a distance of 20 miles. At the point y 1 a pipe 
Fig. 25.— Columnar Dolerite of Depot Nunatak. 
* A. Geikie, ‘ Ancient Volcanoes of Great Britain,’ 1897, vol. ii. p. 120. 
