viii VATU KAISIA 115 



a rope-ladder, to accomplish the last hundred feet of the summit ; 

 but the general uniformity of structure was evident. No detrital 

 rocks came under my observation. 



That the porphyritic basalt represents a later flow around this 

 old andesitic mountain is indicated amongst other things by-this 

 absence of tuffs and agglomerates. Vatu Kaisia is undoubtedly 

 the core of an ancient cone of hornblende-andesite, and as in the 

 case of Mount Soloa Levu, which is formed of somewhat similar 

 andesites (see page 103), it has been more or less completely sur- 

 rounded by later basaltic flows. Vatu Kaisia and Soloa Levu 

 occupy similar positions with respect to the great basaltic table-land 

 of Wainunu, the first lying just within its eastern border, the second 

 lying partly within its western margin. 



The structure of the ridge immediately west of Vatu Kaisia 

 lends support to this view of the formation of this region. The 

 ridge is here, it is true, elevated a hundred feet or so above the 

 table-land which is about 1,000 feet above the sea ; but whilst on 

 its slopes facing the mountain the same porphyritic basalt prevails, 

 there is a limited exposure on its top of the same rock (sp. gr. 2 '68), 

 differing only in the larger size of its porphyritic crystals of 

 hornblende and rhombic pyroxene. 



The narrow gorge isolating the mountain on the west is occu- 

 pied by a tributary of the Yanawai River. It has a depth of 400 

 feet below the ridge ; and as illustrated in the section below it has 



Vatu Kaisia 



1380. 



Basaltic'Plateaii of 

 W 



11 Basaltic 'Rocks . Heights in Feet above, sea 

 EE3 Dacitic Rocks. Length of section 3 miles. 



evidently been largely formed by the eroding agency of the stream. 

 However, at the bottom of the gorge there is exposed a heavy 

 aphanitic basalt showing no olivine and having a specific gravity 

 of 2*85. Though of much finer texture, the felspar microliths only 

 measuring '05 mm. in length, it differs conspicuously from the 

 overlying porphyritic basalt in possessing little or no interstitial 



I 2 



