84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Toneatdsu. — Tongatabu is an irregular crescent-shaped island 22 
miles long, situated in latitude 21° 10'S., longitude 175° 10’ W.! The 
cliffs on the south coast rise to a height of 250 feet, and then fall 
towards the north coast to 10 or 20 feet. Round the coast three 
terraces are seen, and the interior is partly occupied by a shallow 
lagoon. 
Chemical. — The only limestone examined was from Mount Sion Hill, 
at a height of 50 feet, and contained only 2.3 per cent of magnesium 
carbonate. 
Calcium Magnesium 
Height. Carbonate. Carbonate. 
50’ 97.7 2.3 
Microscopical. — The matrix of the rock consists of ‘‘mud.” Cavities 
in the rock are lined with calcite crystals, and the organisms are to a 
large extent represented by calcite infillings. 
Vavau. — At the northern end of the Tonga Group the elliptical 
plateau of Vavau? occurs in association with a number of smaller 
islets. Three or four limestone terraces are seen at levels of 140, 260- 
350, and 420-520 feet respectively. The northern edge of Vavau rises 
to well over 500 feet, and slopes down inland and to the south. 
Chemical. — Limestones from the third, second, and first terraces 
exhibited a close similarity in composition; in each case only about 
3 per cent of magnesium carbonate was found. 
Calcium Magnesium Insoluble 
Height. Carbonate. Carbonate. Residue. 
3d Terrace Highest Point 97.4 - 26 
360’ 97.0 3.0 .03 
2nd Terrace 350’ 97.4 2.6 
310’ 96.8 3.2 
1st Terrace 140’ 97.2 2.8 
Microscopical. Highest point, north shore. — The section is entirely 
filled by an aleyonarian showing spicules whose cavities are lined with 
calcite. 
360’. The rock is made up of organisms and organic fragments, 
lined with an investing layer of fibrous calcite and cemented into a 
compact rock with a clear mosaic of calcite crystals. The organisms 
1 American Journal of Science, 1900, p. 193; Mem. Mus. Comp. Zodl., 1902, 
Vol. XXVI., No. 1, p. 32. 
2 American Journal of Science, 1900, p. 198; Mem. Mus. Comp. Zodl., 1902, 
Vol. XX VI. No. 1, p. 33. 
