ANDREWS : LIMESTONES OF THE FIJI ISLANDS. 33 



of volcanism, probably in Tertiary time, during which masses of volcanic 

 material were heaped up along a fairly well-marked north and south line 

 of weakness. The highest of these masses of volcanic rock, such as 

 Yathata, Tuvutha, and Vatu Vara, came within the reef-building zone 

 of corals, and reefs commenced to form upon them. Alternating epochs 

 of upheaval and stable equilibrium followed, during which the reefs grew 

 outwards over calcareous banks as well as the bedded limestone. This 

 is demonstrated by the long lines of erosion cut into the clitfs of Lau 

 and the sub-horizontal terraces above them. This state of unrest must 

 have continued, for a long period of time, to have produced the vast 

 masses of limestone like those of Mango and Tuvutha. 



Another volcanic phase occurred in recent time. The andesite out- 

 bursts, which disrupted the old reef rock and bedded limestones, formed 

 the coarse agglomerates with large included lumps of coral, and capped 

 the latter with domes of lava. Many of the limestone islands became 

 centres of eruption. At first numerous explosion craters were formed, 

 and blocks of andesite and reef limestone were hurled from the vents 

 until large islands were piled up by the ejectamenta. The former island 

 of Vanua Mbalavu was almost completely wrecked as a result of the 

 volcanic explosions. 



Another instructive feature in these outbursts is the fact that the 

 bases of the great limestone cliffs became in many places the centres of 

 disturbance. If the eruption was extensive, then the cliff vanished and 

 a heap of calcareous agglomerate was all that was left to attest to its 

 former existence. If the disturbance was smaller, then the volcanic 

 matter welled up from the cliff base without wholesale fracturing of the 

 limestone. 



Subsequent to, or pei'haps immediately after the first paroxysmal out- 

 bursts, the andesites welled up in dome-shaped form and buried the coral 

 agglomerates, produced by the earlier explosions, as well as the surround- 

 ing limestone. A final phase in the eruptions is marked by the later 

 eruptions of olivine basalts. These found vents at the bases of the older 

 andesite domes, and at Mango are represented by small tongue-shaped 

 flows and minor masses. 



From these fragmentary traces of basal rocks and from these indubi- 

 table signs of elevation, it is possible to understand the mode of formation 

 of the Lau limestones and their subsequent elevation. Along the sub- 

 marine plateau and running a little west and east of a meridional line, 

 volcanic masses were erupted along the axis of regional elevation ; and 

 on these as a base great accumulations of volcanic ash and conglomerates 



VOL. XXXVIII. — NO. 1. 3 



