74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
calcite. These centers are sometimes crystalline and rhombohedral in 
shape, but often consist of irregular inclusions of muddy calcite. 
20!. A cavernous dolomite whose appearance suggests that most of 
the organisms originally present in the rock have been subsequently 
dissolved. Lithothamnion is still recognizable, although completely 
dolomitized. 
6’ A. A longitudinal section of a dolomitized coral is seen, showing 
many mud-floors and tubes of alge. Many of the dolomite rhombs have 
an outer layer of calcite optically continuous with the dolomite. The 
larger cavities in the coral have been subsequently filled with broad 
crystals of calcite. 
6’ B. Chiefly consists of a dolomitic cement with sections of Litho- 
thamnion and echinid spines, which, though dolomitized, still preserve 
their structures. Cavities are lined with a deposit consisting of four 
successive layers alternately dolomite and calcite, the last layer forming 
broad calcite crystals completely filling the remainders of the cavities. 
6’ C. This dolomitic limestone contains few recognizable organisms. 
Meandrine forms occur in a dolomitic “‘mud” ; Amphistegina and Orbi- 
tolites are very much altered, or represented only by casts, while all 
stages in the disintegration of Lithothamnion can be seen in this section, 
the gradual invasion of the organism by dolomite taking place from 
without inwards, so that in some fragments the outer margin of the 
organism is obliterated, while the central part appears quite unaltered. 
NeILLanciLLan. — Ngillangillah is a small island near Vanua Mbalavu, 
in the Exploring Isles, in latitude 17° 10’S., longitude 179° 2’ W. It 
is entirely composed of elevated limestone, and reaches a height of 
510 feet. 
The island appears to consist of masses of coral reef or reef débris, 
with a bedded limestone underlying the coral rock, not exposed in this 
island, but seen in Bai Vatu, a few miles to the south. There are traces 
of three or four separate upheavals visible on the island.? 
The limestones examined were collected in vertical sequence from one 
of the cliff faces. Mr. E. C. Andrews is of the opinion that the speci- 
mens collected near the sea level belong to younger fringing reefs formed 
at a late stage of the movement of elevation. 
Chemical.— An inspection of the tabulated analyses or the appended 
diagram will show that the composition of the island from top to bottom 
is remarkably uniform. All the limestones are dolomitized, and the 
1 Bull. Mus. Comp. Zodl., 1899, Vol. XXXIIL, p. 90. 
2 Bull. Mus. Comp. Zodl., 1900, Vol. XX XVIII., pp. 28 and 31. 
