CHAPTER XV 



CAINOZOIC 



LAVA AND TUFFS OF EREBUS 



These are described in detail by Dr. H. I. Jensen in Part II. of this Memoir. He 

 classes these as follows : — 



1. Trachyte. 



2. Acid kenyte. 



3. Intermediate to basic kenytes. 



4. Trachydolerites (porphyritic basalts with alkaline phonolites). 



5. Leiicitophyres, tephrites, and basanites. 



6. Basalts without olivine. 



7. Olivine basalts. 



8. Limburgites, magnesian basalts. 



9. Magnetic basalts. 



The trachytes are most typically represented by the phonolitic trachytes of 

 Mount Cis. These contain included fragments of orthojihyre and sanidinite, as well 

 as pyroxene granulites, diabase, and fragments of metamorphosed sandstone. The 

 last mentioned is, in our opinion, probably derived from underlying masses of Beacon 

 Sandstone. The felspars are mostly anorthoclase and soda-sanidine. Leucite, 

 sodalite, interstitial nepheline (?), and analcite are also present. Ferro-magnesian 

 minerals are represented by aegirine-augite with some cossyrite-like hornblende 

 and a little riebeckite. 



From a parasite cone he describes fragments of oligoclase trachyte included in 

 kenyte breccia. This is formed essentially of anorthoclase, augite, and nepheline. 



He describes the rocks of Observation Hill near Hut Point as kaersuetite (?), 

 aegirine-augite-trachyte. He refers to a phonolitic trachyte from Inaccessible 

 Island composed of anorthoclase, albite, pyroxene, analcite, or nosean, and much 

 dark glass. Most of these trachytes contain a little olivine. 



For the trachydolerite and kenyte group he proposes three divisions, viz. 

 (l) acid kenytes, basic kenytes, and trachydolerites. It is this group, characterised 

 by rhomb-shaped or almond-shaped large phenocrysts, mostly of anorthoclase in 

 a dark ground mass, which gives its distinctive character to the volcanic system of 

 Mount Erebus. Certainly the surface rocks of Ross Island are dominantly formed 



