J. J. H. Teall i^ E. T. Newton — Rocks from Tonga Islands. 151 



III. — Notes on a Collection of Eocks from the Tonga Islands. 



By J. J. H. Teall, F.R.S., and E. T. Newton, F.E.S. 

 (Communicated by permission of the Director-General of the Geological Survey.) 



A SMALL series of rocks collected at several localities in the 

 Tonga Islands by Lieutenant J. A. Waugh, R.N., during 

 the voyage of the " Penguin " in 1895, has been forwarded to the 

 Geological Survey for examination, by Admiral Wharton, R.N.,F.R.S., 

 Hydrographer to the Admiralty. As in some particulars these rocks 

 supplement those already described by Mr. A. Harker (Geol. Mag. 

 1891, p. 250), from tlie same group of islands, it seems desirable 

 that a short account of them should be placed on record. 



The specimens first to be noticed were obtained from Mango 

 Island, one of the most central of the Tonga or Friendly Islands, 

 where some of Mr. Haiker's rocks were also collected. It appears 

 from the information supplied with the specimens, that at the south- 

 eastern extremity of Mango Island there is a series of small cliffs, 

 and from these, at a height of six feet above high-water, three 

 types of rock were collected. 



No. Ia (F. 311). This is a fine-grained and bright-green, indurated 

 mud, which does not easily disintegrate in water. Under the micro- 

 scope the rock is seen to be largely composed of irregular, and often 

 angular, fragments surrounded by narrow, deep-green borders. It 

 is possible that these fragments may represent lapilli of volcanic 

 glass, and that the green substance may be of the nature of 

 palagonite ; but the evidence for this is by no means satisfactory. 



No. 1b. Fragments of light-yellowish and reddish decomposed 

 andesitic tuffs. 



No. lo. This is a creamy-white, fine-grained, and chalky-looking 

 rock, similar in appearance to that noticed by Mr. Harker (loc. cit., 

 p. 255) ; it is very calcareous, and when examined as powder, in 

 water, under the microscope, is found to contain numerous sponge 

 spicules ; but the greater part of the rock is made up of amorphous 

 calcareous particles. 



No. II. This specimen was likewise obtained from Mango Island ; 

 but from its highest point, 139 feet above high- water; it is a coarse, 

 decomposed, andesitic agglomerate. 



Both the green mud (Ia) and the white rock (lo) appear to have 

 been deposited under water, and although they are still only six feet 

 above high-water, it is probable that they have been upheaved since 

 their deposition. 



No. ill (F. 312). Two specimens from Tanoa Island, north-east 

 of Mango, were obtained at 20 feet above high-water. One of these 

 is a pale-brown andesitic tufli", and a microscopic section of it shows 

 crystals of green augite and felspar, but evidently mixed with 

 "coral sand," for it includes many fragments of organisms, such as 

 corals, sponge spicules, foraminifera of several kinds, and possibly 

 Eadiolaria. Among the foraminifera, Globigerina seems to be the 

 most abundant ; but the first to attract attention, even in a hand- 

 specimen, are some spherical forms (a large one is 6 mm. in 



