36 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The rock of the second, third, and higher elevations has generally 

 undergone alteration (induration) to a depth of which the limit cannot 

 be ascertained by blasting, and this induration ha3 been more extensive 

 than that which has affected the lower and newer limestones, and has 

 spread through the coral reefs proper as well as through their associated 

 coral debris. At the 300 feet level on the sides of Ngillangillah, an ele- 

 vated reef mass 50 feet thick and consisting of corals, such as Porites and 

 Montipora, shows that the pores of the corals have been almost wholly 

 obliterated through solution and secoudary addition of lime. At the 

 higher levels on Kambara, the coral structures have changed to calcite 

 in many places, and lie in a crumbling calcareous base. But in most 

 cases all coral structure has vanished, the spaces formerly occupied by 

 the corals being represented by dome-shaped cavities (Plate 37). 



At Vatu Vara a white dolomitic rock is filling the pores of the corals, 

 and they are gradually disappearing. This dolomitization is seen at 

 different levels in the Lau islands. Thus at Mango, 240 feet above 

 high-water mark, numerous patches of white rock are found which 

 hardly effervesce with acid. The dolomitization observable on the 

 slopes of Vatu Vara is most pronounced, and its mode of occurrence 

 suggests that it took place from the cause above mentioned during 

 long pauses in the elevating movement ; but the apparent absence of 

 such dolomite patches in the present lines of beach erosion is opposed 

 to the supposition of the coincidence of dolomite belts with old beach 

 lines, as the same dolomite may have resulted from the absorption by 

 the limestone of magnesium salts from sea-spray. 



Another factor in rock alteration is the development of calcite along 

 the higher levels. This may take the form of banded, colored calcite, 

 filling large gaps, which is so common a feature in the older rocks of 

 Mango, or it may take the form of transparent rhombohedra, about one 

 inch in diameter. On the old, undisturbed raised reef flats, numerous 

 patches of this calc-spar formation may be noticed. They are very com- 

 mon on the sub-level summit of Mba Vatu. They occur at Mango less 

 frequently, while in Naitamba we find the finest calcite crystals ; they 

 frequently exhibit a radial disposition, and occur in numerous patches, 

 and are almost invariably connected with old reef flats or lagoon areas. 



From the foregoing considerations, it seems reasonable to infer that 

 the agents which produced the hardened shell of the lower limestone 

 cliffs of Lau and those which formed the secondary ferruginous calcareous 

 rock of the beach-erosion levels and compacted the " beach rock," are 

 similar if not identical ; and these, together with the more pronounced 



