20 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
stone could be examined. Its general appearance was that of calcareous 
patches found on the modern reefs, containing no whole corals, but full 
of small fragments of reef organisms set in a fine, hard, compact white 
matrix. 
Thithia (Plates 18-21).— Thithia and Mango might be studied in 
detail with almost equal advantage. Whatever statement holds good 
in the description of Mango is equally applicable to Thithia. Thithia is 
a trifle larger, being fourteen miles in circuit and four miles in diameter. 
Both have the same vertical cliffs of limestone, the same forest-clad lime- 
stone slopes (Plates 18, 20, 21), and the same uniformly inclined lava 
rock bursting to the sea. Mango is not so completely overwhelmed by 
volcanic material as the sister island, twenty miles away, and shows the 
limestone of the interior more nearly approximating to its former state 
than is the case with Thithia. Mango exhibits terrace formations and 
unmistakable signs of a very recent uplift, while at Thithia only traces 
of terraces can be detected. Formerly Thithia possessed an outer ring 
of raised coralline limestone origin, with a central hollow (Plate 19). 
That hollow has been so encroached upon by lava outbursts that there 
is at present but a hint of its former extent. Thithia might now be 
described as a circular island, indented slightly at one end, and sloping 
steeply to the water, and with four or five bold black cliffs and forest- 
clad rocks to break the continuity of the volcanic slopes. These lime- 
stone rocks at times project but slightly through the down-like slopes 
of lava that have almost buried them. The development of rugged cliffs 
and isolated calcareous blocks comprising the broken country, hereto- 
fore mentioned, represents the survival of the old limestone perimeter 
(Plate 19). 
To the north are evidences of three platforms, respectively 400, 250, 
and 50 feet above tidal influence, the lower one being particularly well 
marked, with the middle one rising above it precipitously. 
Nevertheless from this chaos we can restore the former condition of 
Thithia. It finds its parallel in Mango. Untouched in the volcanic 
uplift, the gap through which the ancient sea ebbed and flowed to the 
inner lagoon still exists (Plates 20, 21). Its 300 yards’ wide entrance 
is protected on both sides by bold rocky scarps 400 feet high, and the 
whole line of gap may be traversed for nearly three quarters of a mile 
inland (Plate 20), with no part more than four feet above high-water 
mark. At its head lie great boulders similar in appearance to those 
disposed along the modern shore lines of Lau. As with the modern 
shore rocks, so here at the terminus of this old sea-way (Plate 21), their 
