XVI UNDU PROMONTORY 233 



lava-rock of which the Vunikondi beds are composed, but scori- 

 aceous and containing more glass in the groundmass. In the 

 hand-specimen beside me, the steam cavities are of all sizes, 

 from that of a pin-prick to a third of an inch (8 mm.) and are 

 generally elongated. 



A careful search of the tuff-deposits in this part of the Undu 

 promontory ought to result in the discovery of remains, both ot 

 plants and of marine mollusks. Mr. Chalmers informed me that 

 fossilised tree-trunks occur on the coast near Vunikondi ; but I was 

 unfortunately not able to discover them. 



Brief Summary of the General Characters of the 

 Undu Promontory from Thawaro and Tawaki to Undu 

 Point. — One suggestive negative feature of this region is to be 

 found in the absence, as far as I could ascertain, of any trace of a 

 crater. Here also, as in the area of acid rocks extending w^estward 

 along the north side of the island to near Lambasa, hot springs are 

 not to be found. The prevailing rocks are in the first place the 

 pumice-tuffs, w^hich not only as a rule form the coast-cliffs, but 

 occur inland as high as the summit of Mount Thuku almost 1,300 

 feet above the sea. They were probably in great part ejected 

 from sub-aerial vents, though no doubt, as in the vicinity of Undu 

 Point, they were often deposited under the sea. The acid and 

 basic tuffs in the vicinity of Tawaki and Thawaro are, as I imagine, 

 largely derived from marine degradation. Next to the pumice- 

 tuffs, massive quartz-porphyries and oligoclase- trachytes are the 

 characteristic rocks. They are probably in most cases intrusive, 

 and present themselves sometimes as vertical columnar dykes, 

 evidently of considerable dimensions. 



The basic rocks, however, are not unrepresented. They occur 

 in one or two places as agglomerates, as in the vicinity of Mount 

 Thuku on the north coast and near Undu Point ; whilst they form 

 " flows " overlying the pumiceous tuffs at Vunikondi. Occasional 

 blocks lying on the surface on the north coast are indications of 

 dykes. The basic rocks, nevertheless, take a very secondary part 

 in the composition of the Undu Promontory. They are in most 

 cases to be referred to the augite-andesites with a specific gravity 

 26 to 277 ; but some, as in the case of those forming the agglom- 

 erates of Thawaro Peak, are hypersthene-augite andesites with 

 specific gravity of about 2-5. Olivine-basalts are not represented. 



