436 SCOTT'S LAST EXPEDITION 
few analogies to be found in the world. 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 cliff 
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 character, 
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 Beacon 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 
