90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
eastern face is the island of Thikombiai lau. On the southern face are 
Ngillangillah, then follows the principal island of the group, Vanua 
Mbalavu, the western shore line of which is hollowed out by a deep 
indentation forming the western lagoon. Off its southern extremity is 
situated Malatta, next Susui, and finally the volcanic island of Munia. 
Vanua Mbalavu is fourteen miles long, forming a sharp angle at its 
highest and broadest point, where the island is nearly three miles wide 
and reaches a height of 930 feet. 
The central and highest part of Vanua Mbalavu is volcanic, but 
towards the northeast and the south the volcanic rocks have lifted up 
an ancient elevated reef which extends from opposite Avea westward. 
The volcanic rocks (Plate 72) dip very rapidly to the north, so that 
less than half way from Koro Mbasanga to Blackswan Point the bluffs 
are elevated limestones. These bluffs are deeply undercut, their surface 
pitted and thoroughly honeycombed, and full of potholes. The extent 
ISLET OFF NGILLANGILLAH. 
of the wearing of this coral rock is well shown by the deep bays and 
numberless indentations which characterize the northern coast of Vanua 
Mbalavu. When we come to the vicinity of Blackswan Point, the 
denudation and erosion of the elevated limestone has reached its 
maximum. Off the end of the large island lies the little island of 
Ngillangillah (Plate 73), which is entirely composed of elevated lime- 
stone, which here attains a thickness of 510 feet. Beyond Ngillangillah 
there are numerous negro-heads and coral patches, some of them 
being of considerable size; they are of ancient coral rock. The coral 
heads and negro-heads extend along the whole length of the outer reef 
