GEOLOGICAL OHSF.HVATION'S LV FIJI. 105 



about the coral heads. If the rocks are especially compact and do not 

 permit the free circulation of aerated waters, the gray colors of the 

 marl may be preserved. Examples of this were found at Laml)asa. 

 The depth of the residual, brick-red soil which overlies the lime- 

 stone is important as an indication of the amount of limestone lost 

 by solution. For example, the depth of soil-cover at the summit of 

 the limestone plateau of northern Vanua Mbalavu averages 2 to 3 

 feet. The average analyses of 17 limestones from the same locality 

 show less than one tenth of one per cent of insoluble matter, (Skeats, 

 1903, p. 75). It follows that the amount of limestone lost by solution 

 must be reckoned in hundreds of feet. 



IGNEOUS ROCKS. 



The following descriptions of the main igneous types refer to the 

 various islands in the order: Viti Levu, Mbengha, Yasawa group, 

 Vanua Levu, Taviuni, Lau Islands, and Kandavu. The petrologic 

 problems involved will be briefly treated later. 



It should be noted that in this paper the word "basalt" refers to a 

 rock composed chiefly of augite and basic, plagioclase feldspar. 



I. Rocks from Viti Levu. 



1) Tanalite. Tonalite outcrops at Vunatoto, as low, rounded hills 

 along the course of a branch of the Singatoka river, leading south from 

 the village of Wai Mbasanga, central Viti Levu. 



The rock is a holocrystalline, light, bluish-gray diorite of medium, 

 coarse grain. It consists largely of feldspars 8 to 10 mm. long and 

 6 to 7 mm. wide, among which are disseminated irregular bits of 

 quartz, hornblende, and biotite. A Rosiwal measurement gave the 

 following proportions by weight: 



Quartz 20 . 42 per cent 



Plagioclase 73 44 " " 



Magnetite 2.02 " " 



Biotite 1.28 " " 



Hornblende 2.84 " " 



100.00 " " 



