440 THE GEOLOGY OF THE XEW HEBRIDES, 



grow it extends outward on a platform of foraminiferal and «hell 

 limestone containing frequent blocks of coral broken off from 

 above. In this way. owing to the steep shore-slopes, a face of 

 coralliferous limestone 300 ft. or 400 ft. in height may Ije rapidly 

 built up. 



vi. Later Basic Ekuptions. — The extrusion of the basic la\as 

 composing the islands of Nguna, (Plate xvi. fig. 1) Mau (Plate xv.) 

 and Pele is here made the subject of a separate paragraph, but 

 must be regarded only as a later phase in a sequence of events 

 beginning with the accumulation of the thick series of agglomer- 

 ates on the mainland. 



The lava-types are quite similar in both localities, but the 

 outlying islands undoubtedly remained active long after the 

 mainland had become extinct. Proof of this latter fact is 

 evidenced by the well preserved shape of the cones and the 

 absence of limestone at any considerable elevation on Nguna, 

 Mau and Pele. On the mainland three distinct coral limestone 

 terraces are distinguishable (Plate xiv.) overlying the agglomerate 

 beds (their growth therefore post-dating the basic eruptions of 

 the mainland) and dipping down towards the east; no trace of 

 these is observable on the lava-slopes of the neighbouring islands, 

 which fact seems to show that ele\ ations of the mainland were 

 contemporaneous with activity in the latter localities. 



The rocks of this series are almost all solid lavas, basalts and 

 basalt porphyries (Sec. 3, §7). At the southern extremities of the 

 islands only is there any notable development of raised coral, and 

 that of limited extent and elevated a few feet. 



vii. River Alluvials. — Owing to the soft nature of the sub- 

 marine tuffs so extensively developed on Efate, river-erosion is 

 comparatively rapid, resulting in the formation of considerable 

 areas of alluvial flats at the extremities of the larger streams. 



The soil composing these flats contains, in addition to strippings 

 from the tuffs and agglomerates of the hills, a large proportion 

 of marine material admixed with it, indicating that deposition 

 took place largely in sea water, and that consequently recent 

 upheaval must be responsible for its present elevated position. 



