CHAPTER XVII 



THE VOLCANIC ROCKS OF VANUA LEVU 



The varied character of the volcanic rocks in my collection is 

 brought out in the following Table, where I have grouped about 

 400 rock-sections, excluding those of the tuffs and finer detrital 

 deposits. The small proportion of plutonic rocks should be noted. 



Olivine-basalts 23 per cent. 



Augite-andesites 40 „ ,, 



Hypersthene-augite-andesites 17 „ ,, 



Acid andesites, including hornblende and quartz- 



andesites, &c 12 „ „ 



Oligoclase-trachytes, quartz-porphyries or rhyolites . 6 ,, „ 



Hypersthene-gabbros and diorites 



j> 



100 



In order to avoid the necessity of frequently describing rocks 

 of the same type it has been found requisite to devise a method of 

 classification. In carrying out this somewhat laborious task I have 

 often been surprised at the readiness with which rocks, the relations 

 of which had been previously very difficult to ascertain, fell into 

 their place in the scheme. Although many of my uncertainties 

 have been thus removed, a large number of doubtful points remain. 

 I venture to think, however, that others may be able to employ and 

 also to extend the method of classification here employed. 



The general plan followed has been worked out in detail for the 

 olivine-basalts and the pyroxene-andesites of the more basic type ; 

 whilst lack of materials has prevented its further elaboration in the 

 case of the acid andesites, oligoclase-trachytes, quartz-porphyries, 

 &c. The treatment of all the classes has been uniform, the scheme 

 being the same whether applied to a basalt or to a dacite. 



In describing the general method I will take the Augite class. 

 As will be seen in the Synopsis that follows, this class is first 



