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 has been more extensive 
than that which has affected the lower and newer limestones, and has 
spread through the cural reefs proper as well as through their associated 
coral débris. 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 secondary 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 
