GEOLOGICAL OBSERVATIONS IN FIJI. 119 



4. Rocks from ihe Divide between, the Lamhasa and Savu Savu Bay' 



Districts. 



a. Basaltic Conglomerate. The crest of the divide between Lam- 

 basa and Savu Savu Bay is formed of conglomerate rocks containing 

 rounded pebbles of basalt 1 to 2 cm. in diameter. Some of the pebbles 

 are green, others slate or brick-red color according to their degree of 

 alteration. Their structures vary from porphyritic through glassy 

 to scoriaceous. They are set in an ashy paste. Lower down on the 

 slopes of the ridge, the paste becomes calcareous and the pebbles, 

 smaller. Foraminiferal tests and even small shells are sometimes 

 found. It would appear that the present crest of the ridge is not 

 far from the strand-line of the ancient volcanic period. The sedi- 

 ments dip gently to the southwest. 



The microscope shows that all the pebbles of the conglomerate are 

 basaltic. The vesicles of the rocks from the crest are filled with, 

 chalcedonic amygdules while the lower rocks have calcareous fillings. 

 Olivine and hypersthene are the prevailing ferro-magnesian minerals; 

 the feldspar is a medium to basic labradorite. 



b. Gabbro Porphyrite. Descending the crest of the divide on the 

 way from Savu Savu Bay to Lambasa, the first 500 feet is very 

 precipitous. Thereafter the paths along the stream beds are gentler. 

 Near the intersection between the two slopes there are waterfalls 

 at certain dikes which resist erosion better than the adjoining con- 

 glomerates. Specimens of the dikes were collected from an altitude 

 of 1100 feet and 800 feet. They proved to be very similar. A speci- 

 men from the lower level will be described. 



The slightly vesicular rock is iron-gray in color, showing phenocrysts 

 of feldspar, 3 to 4 mm. in length, evenly disseminated in a finely 

 crystalline, felty ground-mass. 



Feldspar phenocrysts are very abundant. Very few augite crystals 

 are present. The ground-mass has a typical diabasic texture of coarser 

 grain than the average basalt, hence the rock is referred to as a gabbro 

 porphyrite. The feldspars are usually not zoned but are in laths 

 (as labradorite, Abso Anso). One or two zoned specimens were found 

 having the composition: — 



Outer zone, oligoclase, Ab75An25, 

 Middle zone, andesine, AbssAnjO} 

 Inner zone, labradorite, Ab43Aii57. 



