RELATIVE AGES OF THE VOLCANIC ROCKS. 
123 
Africa.* In the same year similar ideas were brought forward with greater elaboration 
by Becke in a striking contrast which he drew between the volcanic rocks of the 
Bohemian Mittelgebirge and those of the Andes, j He suggested that these two Pacific 
and Atlantic types were intimately connected with the two tectonic processes of Suess 
which have mainly affected the earth’s crust, viz., faulting by tangential pressure, and 
fracture by radial contraction ; where young volcanic rocks occur along a faulted 
mountain-chain like the Andes, they belong to the Pacific group ; where, on the other 
hand, they occur on block-fractures (“ Schollenbriiche”), they belong to the Atlantic type. 
In the case of these Antarctic eruptions, we have volcanic rocks of undoubtedly 
Atlantic type developed along a coast which has been described as typically Pacific. J 
They form, therefore, apparently an exception to Becke’s rule ; but it may be pointed 
out that the volcanoes of South Victoria Land, from which specimens of lava have been 
obtained, are not generally ranged parallel to the coast, but appear to occur along lines 
of weakness directed nearly at right angles to it. § 
As regards the relative ages of the basalts and alkali-rich rocks of the Boss 
Archipelago, the observations in the field lead to no very conclusive result. From the 
intrusive appearance of the trachyte on the south-east end of Black Island ( see p. 14), 
Mr. Ferrar was inclined to regard the trachytes as younger than the basalts, but 
he was unable to examine closely the actual junction of the two rocks at this 
spot. In the case of the kenytes of Mount Erebus and the islands in Erebus Bay, 
Mr. Ferrar is of opinion that they are older than the basalt-flows of Winter Quarters, 
since on Kazor Back Island they are bent into a sharp anticline which has not affected 
the other rocks. It seems probable, therefore, that the trachytic rocks having a 
chemical composition so similar to that of the kenytes are also older than the basalts. 
This idea is also supported by the fact that the bombs scattered over trachytic rocks 
are of the same character as the basalts. Mr. Ferrar also describes the trachyte-bosses 
(e.g., that exposed in the Gap) as occurring like “islands,” without off-shoots into the 
basalt. Thus Observation Hill is probably not intrusive in the basalts, but stands out 
from them as an older rock, just as the phonolite-peak of Fernando Noronha projects 
above the younger basalts. 
On the whole, therefore, it seems probable that in these Antarctic volcanic rocks 
the sequence of eruption has been from rocks of medium basicity to basic, and that the 
trachytes and kenytes preceded the basalts. 
* Prior, Contributions to the Petrolog. of British East Africa, etc., Mineralogical Magazine, 1903, vol. xiii, 
p. 260. 
f Becke, Tschermak’s Min. Petr. Mitth. 1903, Bd. xxii, p. 248. 
| Gregory, Nature, 1906, vol. lxxiii, p. 300. 
§ See, however, footnote on p. 140. 
R 2 
