32 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



with ferric oxide. A small hole exists near the present sea-level in 

 the limestone, and by crawling in one finds a small cave about 8 feet 

 long and 3 feet wide. The sides are composed of red volcanic clay 

 most distinctly bedded. Above this is found a reddish, hard lime- 

 stone, and a very short distance away the coral rock itself occurs. Hot 

 springs also exist throughout this limestone area. 



Mango. — On the summit of a typically shaped andesite dome, and 

 carried to its present position by the eruption of this same lava mass, 

 a massive bedded limestone occurs (Plate 2, Fig. 1). It overlays a 

 greatly decomposed volcanic conglomerate, gray to greenish in color. A 

 number of gasteropods are scattered throughout the mass of the con- 

 glomerate. The stratified beds immediately above the decayed vol- 

 canic material are a dense, compact, reddish limestone, with fragments 

 of fossils in places. It is very similar to the basal limestone at Mba Vatu 

 and Malatta. 



The bulk of this volcanic conglomerate cannot lie at a depth much 

 below sea-level, if, as seems probable, its thickness is comparable with 

 that of the outlier shown on the left hand of the section of Mango given 

 on Plate 2, Fig. 1, which has been pushed up into its present position 

 by an outburst of andesite. 1 We can easily avoid the necessity for ex- 

 plaining away huge deposits of coral reef origin, since the overlying reef 

 material occupied but a fairly thin crust. 



Tuvuthd is as instructive as Mango. An old volcanic conglomerate 

 almost identical in appearance with that seen on Mango occurs on the 

 sea beach. A newer andesite lava has broken through and poured over 

 it and also towards the limestone cliffs, leaving a gap between the two. 



Geological History of the Lau Group and its relations to 

 the Western Fiji Islands. 



It is now possible to suggest the genera] lines along which the present 

 features of Lau were developed. Calcareous deposits, which afterwards 

 formed the " bedded " limestones, were laid down on the floor of the 

 ocean, the submarine plain which later was to form the foundation of 

 the Lau Group. The existence of a basal volcanic conglomerate below 

 the reef rock at Mango and Tuvutha shows that there succeeded a period 



1 This does not imply that compact basal limestones project above the floor of 

 the central hollow, but that the undisturbed and the disrupted rock are similar in 

 their upper portions. 



