THE GREAT FAULTS 299 
Perhaps the most characteristic feature of the whole of Victoria 
Land is the existence in practically all parts yet visited of a line of 
dark level-bedded rock, which stands out on clifi faces, produces 
pinnacled mountains, and generally dominates the topography. 
This is caused by intrusions of dolerite in the form of a sill, which 
from the district of its first description may be called the 
McMurdo Sill. From Lat. 71° down to Lat. 85°, and probably 
beyond, this dolerite is found, varying only slightly in charac- 
ter, and precisely similar in mode of occurrence. In places it 
occurs as one thick sill, nearly always columnar in form, up to 
1500 feet in thickness, in others it splits into two or more sills 
of smaller size. In general it has intercalated itself between the 
strata of the Beacon Sandstone, but in some cases it has formed 
a sill through granite. In one particular district, that of the 
Ferrar Glacier, it forms a sill of 300 feet almost level-bedded, 
dividing two very different types of granite. Its intrusion was 
for the most part quiet, and has left little effect, beyond a baking 
of the strata in its immediate vicinity. In places, however, it 
was evidently more violent, for huge blocks of granite or Bea- 
con Sandstone are found in it, torn from their parent masses. 
The intrusion of these sills of molten rock probably raised the 
whole area to some extent, and prevented any further deposits. 
The true boundaries of the area intruded by the McMurdo Sill 
have not yet been located, but it can hardly be less than the 
size of the British Isles, and is probably much greater. 
There is one more marked period in the history of South 
Victoria Land. Probably about the middle of Tertiary times 
that part of the crust was subjected to further shrinkage stress 
of an even kind, which ultimately resulted in a series of 
great breaks or faults along the present coast-line. On the 
upthrow side of the fault the land was slowly raised into the 
present plateau, while on the downthrow side the land was de- 
pressed below sea-level, and now forms the Ross Sea and the 
sea-bottom below the Great Ice Barrier. Simultaneously with, 
and probably as an effect of this faulting, there occurred a great 
outburst of volcanic energy along the line of break. At many 
points volcanoes were formed, the chief centres being the Ross 
Archipelago, the Cape Adare Peninsula, and the Balleny Islands. 
This outburst is now just dying out, only two volcanoes being 
still active, Mt. Erebus on Ross Island, and Sturge Island in the 
