122 
G. T. PRIOR. 
In both the basaltic and alkaline groups, the ratio of soda to potash is nearly the 
same, with Na>K. A similar basaltic hornblende occurs in certain rocks of each 
group, and not in others in which its place is taken by olivine. The presence or 
absence of hornblende in both groups probably depended to a large extent upon the 
conditions of eruption ; in the dyke-rocks, which cooled while still under a high pressure, 
the mineral was more likely to survive than in the lavas. 
Altogether, this Antarctic region, from Scott Island in the north to the Minna 
Bluff in the south, has good claim to be regarded as a definite petrographical province 
CaO 
Fig. 67. — Graphical Representation op the Chemical Composition op the 
Olivine-basalt (656) from near the Gap. 
CaO 
Fig. 68. — Graphical Representation of the Chemical Composition op the 
Leccite-kentte (818) prom Cape Royds. 
characterised by the association of basalts of limburgite-type with alkali-rich rocks of 
medium basicity in which anorthoclase is the prevailing felspar. 
In a wider sense, these Antarctic rocks belong undoubtedly to the great Atlantic 
as opposed to the Pacific type of eruption. The marked contrast between the younger 
volcanic rocks of the Atlantic volcanic chain and its lateral branches, and those of the 
volcanoes encircling the Pacific Ocean, was pointed out by the author amongst the 
conclusions drawn from a study of the igneous rocks of the Great Rift Valley of East 
