OF THE ROSS ARCHIPELAGO 



141 



rounded forms within the hornblende along with grains of magnetite. Fig. la shows the 

 normal course of the resorption and Fig. lb an unusual case where it has taken place 

 on one side of the augite only, with the development of the hornblende in parallel 

 position. The hornblende is a common brown variety with pleochroism from brown 

 to yellow-brown, and is perfectly euhedral where it does not impinge on olivine or 

 augite, with the faces M (110), B (010), A (100), and dome terminations. Felspar 

 occasionally forms large lath-shaped crystals comparable in size to the above-named 



Fig. la 



Fig. lb 



minerals, but cannot be considered a phenocryst, since it includes abundantly the fine 

 hornblende needles of the ground-mass. It shows simple albite twins with low extinction 

 angles, but too few opportunities of measurement present themselves to determine the 

 maximum, nor can the refringence be compared with that of balsam since the mineral is 

 always surrounded by analcite. The latter occupies large areas of the ground-mass, of 

 which the chief constituent is hornblende in small prisms crossing one another in all 

 directions. The amygdules are generally completely filled with analcite and calcite, the 

 former being the earlier mineral in sharp crystals (Fig. 6, PL II). 



On the assumption that a calcic felspar has been replaced by analcite and calcite, 

 the rocks agree in all respects with olivine camptonites. 



Origin of the Hornblendic Inclusions of the Trachytes 



Lacroix has found that inclusions more basic than their host, although not so 

 common as inclusions of sanidinites, have nevertheless a wide distribution in trachytes, 

 especially in those rich in ferromagnesian minerals. They are for the most part of the 

 nature of porphyrites. In phonolites the analogous inclusions are generally camptonites, 

 rocks which, he points out, frequently accompany nepheline syenites, the plutonic 

 representatives of phonolites. He considers them, nevertheless, as basic segregations 

 from the phonolitic magma.* 



While the Observation Hill inclusions might easily be considered as segregations, 

 since the same hornblende is so abundant in the trachyte, the Cape Bird camptonites 

 cannot be so regarded. The alteration they have undergone could scarcely have taken 

 place had they never existed but within the trachyte, since the latter is a relatively 

 fresh rock. It is simpler to conceive of them as fragments of true camptonites which 

 were completely consolidated and partly altered before they were enclosed in the 

 trachyte magma. 



The fact that the basic inclusions of the Boss Island trachytes are camptonites and 

 not porphyrites is a further confirmation of the phonolitic affinities of these trachytes. 



* Les Enclaves, etc., p. 416. 



