102 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Niue, Vavau, Tongatabu, Niau, Guam, and Makatea are of this type. 
In the case of Makatea (Metia), however, Dana and Silliman have re- 
corded the occurrence of dolomitic limestone, and it is possible that an 
examination of other specimens may show that some of the islands men- 
tioned do contain dolomitic rocks. Limestones containing little or no 
magnesium carbonate are found associated with dolomitic limestones in 
Christmas Island, Mango, Namuka, and Eua. 
On examining rocks from these islands it is found that dolomitization 
may occur at several horizons. In Namuka, the lowest and oldest rock 
is dolomitized, while in Christmas Island one of the Oligocene or Eocene 
rocks of Sydney’s Dale, from a height of 350 feet, is highly dolomitic. 
Again, at Christmas Island, in the cliff section, east of Phosphate Hill, 
dolomite occurs in the Miocene rocks at a level of 600-640 feet. At 
Mango, limestones are only found at three heights, 370, and 310, and 
298 feet; all the remaining specimens being highly dolomitic. Perhaps 
the occurrence of dolomite is most usual at the highest points of the 
islands. This is found to be particularly true of Christmas Island, 
where the three highest hills are all dolomitized, as are the tops of 
Mango, Ngillingillah, and Vatu Vara, while dolomitic limestones are 
found near to the summits of Yathata and Kambara. 
Very little magnesium carbonate is, as a rule, found in the limestones 
from the newer reefs fringing the raised coral islands. In certain cases 
as much as 10 or 11 per cent of magnesium carbonate is found without 
the formation of recognizable crystals of dolomite, but in only two in- 
stances have highly dolomitic limestones been found among the rocks of 
the inland cliffs or terraces. At Christmas Island, the top part of the 
upper inland cliff south-east of Phosphate Hill is found to be highly 
dolomitic, while the other example is from the first inland cliff or ter- 
race of the island of Eua. But in these cases it is difficult to say with 
certainty whether the rocks belong to a true fringing reef, a ledge in the 
older rocks cut back by the sea during a pause in elevation, or a sub- 
marine talus composed of the débris of the older rocks above it. 
Many of the dolomitic limestones, in composition, approach to, and 
even slightly exceed, 40 per cent of magnesium carbonate. But it is an 
interesting circumstance that, up to the present, no rock having the 
composition of a true dolomite has yet been met with among the 
limestones from. coral islands. The maximum value for magnesium 
carbonate yet recorded is 43.3 per cent, from one of the Christmas 
Island rocks. 
B. More or less extensive deposits of guano are found here and there 
