xxi ACID ANDESITES 



2 95 



If we restrict the term to a hornblende-andesite carrying por- 

 phyritic quartz and displaying a microfelsitic groundmass, such 

 rocks, though they form some of the highest peaks in the 

 Ndrandramea district, namely Ngaingai and Wawa-levu, would not 

 be very frequent in Vanua Levu. If, however, a microfelsitic 

 groundmass is alone necessary to constitute a " dacite," the great 

 majority of the acid andesites of the island would fall under this 

 designation. 1 This has long been a controverted point in petrology. 

 If I adopted the last procedure, my general classification for the 

 andesites would fall into confusion and many rocks without any 

 quartz would be included in the dacites. 



In the Synopsis it will be seen that my classification of the 

 andesites is as far as concerns the great groups based on the 

 mineral and not on the structural characters. There are three 

 sub-classes closely allied to each other, the hypersthene-andesites, 

 the hornblende-hypersthene-andesites, and the quartz-hornblende- 

 hypersthene-andesites or dacites, which cannot be distinguished at 

 their boundaries by their penological characters or by their different 

 modes of occurrence. These groups of rocks which include all the 

 acid andesites of the island will now be dealt with. 



The Acid-Andesites of Vanua Levu 



{Comprising the hypersthene-andesites ; hornblende-Jiypersthene-ande- 

 sites ; and quartz-hornblende-hyperstJiene-andesites or dacites) 



These rocks compose in mass numerous isolated hills that rise 

 up abruptly in the interior of the central portion of the island. 

 Such hills, or mountains, as they might be often termed, usually 

 attain a height of from 700 to 1200 feet above the surrounding 

 country, and possess precipitous slopes and frequently perpendicular 

 cliff-faces. In the geological description of the island, I have re- 

 ferred in detail to these mountains, when speaking of Na Raro, 

 Vatu Kaisia, Ndrandramea, Ngaingai, etc. ; and illustrations of 

 some of them are included in this work. It may, however, be here 

 remarked that they are as a rule rudely conical with rounded or 

 peaked summits. The ground-plan is generally elliptical in outline ; 

 and in consequence the profile often varies from different points 

 of view, so that as in the case of Na Raro, it is that of a sharp conical 

 peak when the mountain is viewed " end-on," or of a broad trlm- 



1 The term " felsitic andesite " is suitable for this microfelsitic type. 



