254 A. HarJcer — Rocks from the Tonga Islands. 



The volcanic rocks of Eua are overlain, with evident unconformity, 

 by massive white limestones, and the earlier group must have suffered 

 considerable denudation before the newer was deposited. This 

 appears from the fact that while the horizontally bedded volcanics 

 crop out in the interior at a height of more than a thousand feet, 

 the limestones come down to sea-level on the coast within a distance 

 of half a mile. Further, the dykes which cut through the volcanic 

 deposits terminate without entering the overlying calcareous 

 strata. 



The limestones occur in three terraces, and Mr. Lister was led to 

 regard them as elevated reefs. He observed reef-corals on the 

 edge of the lower terrace and on the top of the highest one. An 

 examination of a few thin sections of the rocks showed indeed a 

 comparative want of coral fragments, the rocks sliced, with their 

 abundant foraminifera, some of large size, recalling strongly the 

 Orbitoidal Limestones of Sumatra and Borneo as described by 

 Messrs. H, B. Brady ^ and A. V. Jennings^ respectively. When 

 good coral remains occur, they are associated with similar forami- 

 nifera, etc. [1329]. The specimens have, however, been submitted 

 to Dr. John Murray, whose large experience of calcareous deposits 

 is well known, and, pending the results of his examination, it will 

 be sufficient here to notice the more obvious petrological characters 

 of the rocks. 



Some examples, e.g. from Maui's Oven, are rough-textured 

 yellowish limestones, in which abundant foraminifera can be seen 

 on a hand-specimen. In general, however, the deposition of 

 secondary calcite has converted the mass into a white or cream- 

 coloured limestone of very compact appearance, not unlike some 

 travertines, but containing sometimes little irregular vacuities which 

 the calcite cement has not filled. 



The specimens sliced, whether from the lower, the middle, or 

 the upper terrace, show abundant foraminiferal tests and other 

 organic remains which will not be more closely described in this 

 place. Small oolitic growths are common in the secondary calcite 

 cement, but they are invariably of the spherulitic type. Concentric- 

 coated oolitic grains are conspicuously absent, nor is there any trace 

 of clastic material such as quartz- or shell-sand, volcanic detritus, 

 etc. The only large fossil observed is a cast of a CeritMum, not 

 perfect enough for specific determination. 



I pass on to the specimens from other islands composed partly 

 or wholly of volcanic accumulations. The purely coral islands, 

 Tongatabu, Vavau, Nomuka, etc., will not be noticed. 



Mango (Comango on the Chart) appears to be built entirely of 

 volcanic ejectamenta. Mr. Lister found no lavas except as frag- 

 ments in the tuffs. These, in the specimens examined, range up to 

 about six inches in length, and have a dull brown decomposed 

 aspect, with in many cases small vesicles filled with calcite. A slice 

 of one [1267] shows a microlitic andesite with marked flow- 



1 Geol. Mag. (2), Vol. II. p. 532, 1875. 



2 Geol. Mag. (3), Vol. V. p. 529, 



