GEOLOGICAL OBSKia'ATlOXS L\ 11,11. 20 



be seen in crossint;; Vamia Leva from Lamhasa southward to Savu 

 Savu Bay on the southern coast ot" the ishiiid. The hrst part ol" tliis 

 journey leads along the flats of the Lamhasa ri\er. Then an ascent 

 of 125 to 150 feet brings one to a wide plain stretching inland for 15 

 to 20 miles. The rocks underlying this plain are largely vesicular, 

 andesitic lava, seamed with chalcedonio veins and containing amyg- 

 dules of chalcedony and ciuartz. Calcareous ash-beds also occur. 

 Near the top of the ridge forming the central divide of the i.sland, 

 fossiliferous tufTs are formed at elevations from 2500 to 2700 feet. 



The evidence gathered at the coralliferous-limestone exposure led 

 the writer to believe that, following the formation of the valley pene- 

 plain, there was an uplift of SO to 90 feet; then a short period of stabil- 

 ity during w^hich a shore platform was cut in the soft limestones. 

 Later, another uplift of 40 to 50 feet allowed valleys to be carved in 

 the shore platform thus formed. Still more recently a rise of sea-level 

 has allowed 10 to L5 feet of volcanic silts to accumulate in'these valleys. 



A \mt to the divide east of the Lambasa plain and southeast of 

 IVIt. Avuka (see sketch map, Figure 10) showed that other stages 

 could be recognized in the history of the island. The road winds 

 gradually up from the river flats over a series of low lateritized hills, 

 and then follows the course of a small creek (Na Suva) which has 

 carved in the laterite a gorge-like valley, 20 to 30 feet deep. 



At an elevation of 350 feet, the lateritized materials have been 

 washed away and occasionally the true nature of the rocks may be 

 seen. They are glassy lavas, unconformably overlying maturely 

 dissected gabbro. Ascending still further to the divide at an eleva- 

 tion of 750 feet, the gabbros are lost for a time but appear again along 

 the axis of the ridge. They outcrop for 200 yards along the road as 

 it passes over the divide and, south of the road, rise in a dome 1100 

 feet high. 



On the eastern slope of the divide, pitchstone pebbles are asso- 

 ciated with the glassy flow lava which mantles the gabbro surface. 

 To the north. Mount Avuka, 1976 feet in height, is formed of andesitic 

 ash and agglomerate which overlie the glassy flows conformably. 



An outline of the history of north-central Vanua Levu is, therefore, 

 somewhat as follows: — 



(1) An old land of plutonic rocks was maturely dissected and 

 submerged. 



(2) During the great submergence a thick series of volcanic rocks 

 was laid down; first acid flows and later andesitic ash and agglomer- 

 ates. 



