SKEATS : CHEMICAL COMPOSITION OF LIMESTONES. 83 
C. The Tonga Group. 
The Tonga, or Friendly Islands, consist of an elliptical ring of islands, 
of which the most important members are Eua, Tongatébu, and Vavau. 
Eva. — The most southerly island of the Tonga Group is Eua, which 
is situated in latitude 21° 20/S., longitude 174° 55’ W. = The island rests 
on a volcanic base,? and consists largely of elevated limestone with verti- 
cal cliffs on the eastern face, some of which are over 1000 feet in height. 
At all projecting parts are lines of terraces ; at the north three are visible, 
at the south five, and traces of a sixth. The island is composed of two 
ridges ? running north and south,-and separated by a deep valley. The 
western ridge is 500 feet in height, the eastern one 1000 feet or more. 
Chemical. — A specimen collected from the slopes of the central valley 
at 600 feet contained 7.7 per cent of magnesium carbonate, while another 
from the second terrace at 250 feet yielded only 4.5 percent. The third 
specimen was from the first terrace at 120 feet, and was found to be 
highly dolomitic, containing no less than 40.4 per cent of magnesium 
carbonate. 
Calcium Magnesium 
Height. Carbonate. Carbonate. 
600’ 92.3 Teal 
250’ 95.5 4.5 
120’ 59.6 40.4 
Microscopical. 600'. — A crystallized ‘coral sand” and “mud” from 
the central valley of the island. One coral present shows its original 
structure, but the rest are represented by “ ghosts,” consisting of a 
dirt line surrounding clear calcite. The matrix is partly “mud,” partly 
granular calcite. 
250!. Second Terrace. — Fragments of Globigerina, Carpenteria, Hali- 
meda, etc., are set in dark “mud.” The aragonite organisms such as 
Halimeda are recrystallized, as are some of the calcite forms such 
as Globigerina. Carpenteria, however, retains its fresh brown ap- 
pearance. 
120’. First Terrace. — This appears to have been a coral “mud” 
which has been dolomitized. Some of the organisms such as Carpenteria 
and aleyonarian spicules remain apparently unaltered. The arrangement 
of the crystals round some cavities suggests deposition from solution. 
1 Notes on the Geology of the Tonga Islands, J. J. Lister, Q. J. G. S. Vol. 
XLVIL, pp. 590-616. 
2 American Journal of Science, 1900, p. 198; Mem. Mus. Comp. Zodl., 1902, 
XX VIE, No: 1p. 87. 
VOL. XLII. — NO, 2 3 
