A. Sarlicr — Rocks from the Tonga Islands. 255 



structure and much ferruginous decomposition-producfc. The rock 

 is not porphyritic. 



The fragmental accumulations from this island vary much in 

 character. Specimens from the eastern hill are white chalky-looking 

 rocks, in which nothing is to be seen beyond a rare crystal of augite. 

 They are very calcareous, and the residue after treatment with acid 

 is merely a mass of very fine scoriaceous and dusty material with 

 fragments of crystals of felspar, hypersthene (?), apatite, etc. From 

 other parts of the island come fine grey ashes with white spots of 

 decomposing felspar. These are non-calcareous, and consist of 

 minute fragments of brown-stained glass and various volcanic 

 minerals mingled with fine dust. Other rocks unmixed with cal- 

 careous matter are, rather, volcanic sandstones, enclosing numerous 

 fragments, usually rounded, of yellow or brown lava, etc. Specimens 

 of coarser texture — volcanic conglomerates — come from the western 

 or Obsei'vation HilL These have a partly calcareous matrix and 

 include fragments of lava usually an inch or more in diameter and 

 of subangular form. In these coarse accumulations occur blocks of 

 coral, specimens of which are as much as six inches across, and have 

 portions of the conglomeratic rock adhering firmly to them. 



Nomuka-iki is a small island lying to the south-west of Norauka. 

 It is described as consisting in its northern part of a flat of coral 

 sand, while the southern and higher portion is of stratified volcanic 

 ashes, alternately fine and coarse, which are well exposed in a cliff 

 on the west, and contain marine fossils. The specimens from here 

 are mostly yellow-brown or greyish ashes of fine texture, only 

 occasionally showing fragments up to an eighth of an inch long, some 

 of which are of decomposing andesite. Little glistening felspars 

 are visible with a lens, besides countless little black dots which 

 seem to be partly altered pyroxene. Some specimens from the 

 upper layers are rather coarser, and show more conspicuous frag- 

 ments. In these, well-formed prisms of lustrous black augite may- 

 be recognized as well as the felspars. The microscope shows that 

 these latter are, as usual, of a triclinic species ; that a green, 

 pleochroic rhombic pyroxene occurs, in addition to the green 

 augite ; and that yellowish volcanic glass and fine dust are also 

 represented. None of the Nomuka-iki specimens are calcareous 

 enough to effervesce with acid. The only fossil in the collection is 

 a well-preserved Pyrula. It rather resembles the Eocene P, nexilis, 

 but has more delicate markings, and is perhaps to be matched 

 among later forms, 



Tonumeia, a small island situated to the south of Nomuka and 

 Mango, consists, according to Captain Oldham's observations, of 

 stratified volcanic tuffs, which are exhibited in a cliff 80 feet high 

 on the west coast, showing a dip of 3° to the south, A specimen 

 (I.) of the finer tuff is a rather fine volcanic ash compacted by a 

 calcareous and ferruginous cement into a dirty yellow-brown rock. 

 In the residue insoluble in dilute acid are recognized chips of a 

 • monoclinic and a fibrous rhombic pyroxene with a basic felspar, 

 besides the usual brown-stained glassy and scoriaceous fragments. 



