GEOLOGICAL OHSKHVATIONS IN FIJI. 115 



B. DESCRIPTION OF ROCK TYPES COLLECTED BY THE WRITER. 



Tlie writer was able to ^'i.sit but a tew places in Vanua Le\'ii and 

 hence can add little to the work already done by Guppy. The facts 

 concerning rock distribution coincide with those already outlined. 

 Rocks from four districts only will be described. 



1. Rocks from the Mbutha Bay District. 



a. A porphyritic, greenish-gray basalt was collected from a sill in 

 the agglomerates which, with ash-beds, form the country rock of 

 most of the district about Mbutha Bay. The exact locality lies 

 midway between Mbutha and Natewa Bays at an elevation of above 

 1700 feet. 



The feldspar phenocrysts usually have the composition of medium 

 labradorite (Ab4oAn6o), but a few are zoned, showing an outer zone of 

 andesine, (AbssAn^), an intermediate zone of acid labradorite, 

 (Ab45An55), and an inner zone of anorthite (AbsAngs). 



The ground-mass is diabasic. 



b. Ih/persthene basalt was found in a boulder of an agglomerate on 

 the south bank of the Mbutha River, I5 miles from its mouth. The 

 rock is grayish white in color, compact, and finely granular. The dark 

 phenocrysts are chiefly hypersthene; a few small brown hornblendes 

 may be seen. Beautifully zoned feldspars abound. They consist of 

 four layers, an outer of acid andesine, (AbesAnsy), the next of basic 

 andesine, (Ab55An45), which also forms the innermost zone, and a third 

 of basic labradorite, (iVb28An72). 



The ground-mass has a trachytoid texture and consists of feldspar 

 with abundant minute grains of magnetite. There is some evidence 

 of devitrification. 



2. Rocks from the Lambasa District. 



a. Gabbro. Gabbro forms a low, rolling topography along the 

 divide between the Xgawa and Mbutha Sau rivers just east of Lam- 

 basa. The eroded gabbro surface was submerged and covered by 

 marine ash-deposits and basaltic flows; subsequent uplift permitted 

 further erosion to uncover the gabbro. 



The specimen to be described was collected from the crest of the 

 divide, at an elevation of 1500 feet. It is a medium-grained, iron-gray 



