256 A. Sarlcer — Rocks from the Tonga Islands. 



Other specimens from this island are pebbles found on the beach. 

 One (II.) is a coarsely amygdaloidal lava, perhaps not a native rock. 

 Two others are black nodules of irregular but rounded form, the 

 larger being two inches in diameter (HI.)- They have externally 

 a graphitic lustre, while the interior is duller and shows a certain 

 eccentric radial structure. Eough chemical tests indicate some oxide 

 of manganese, and the general characters are those of psilomelane ; 

 but the interior is softer, and the specific gravity of one nodule was 

 found to be only 3-54. This perhaps indicates a partial conversion 

 into wad or some other hydrated substance. These nodules are 

 evidently native to the island, and portions of the calcareous matrix 

 in which they have been imbedded are still adherent on their surfaces. 



Captain Oldham also collected specimens from the small island 

 Tonua (not named on the Chart) situated north-east of Mango. Of 

 these, an "altered coral sand" (IV.) from the summit of the island 

 seems to be free from foreign admixture ; the rest, from a cliff 20 

 feet high on the north side, are volcanic ashes usually with but little 

 calcareous matter, despite the fact that organic remains are visible 

 in several specimens. These stratified volcanic rocks, with an 

 easterly dip, build the mass of the island. Most of the specimens 

 are soft yellow-white rocks with a lumpy appearance, although the 

 enclosed fragments, up to about an inch in diameter, do not differ 

 essentially from the matrix in which they are .imbedded (V. and Va.). 

 Minute glistening crystals of felspar and augite are visible here 

 and there in the dull mass, and the microscope shows only these 

 minerals with an occasional broken needle of apatite, and much fine 

 dusty matter. In other examples from the same cliff the fragments 

 are more distinct, and many of the smaller ones are rolled bits of 

 dark lava (VI. and VII.). These rocks contain numerous organic 

 remains, among which the most conspicuous seem to be the conical 

 tests of Pteropods : a rather indistinct cast of a Gasteropod also 

 occurs — possibly a Murex from the form of its canal, although the 

 varices are not very strongly marked. One specimen is traversed 

 by a half-inch seam of fine, non-calcareous character, with a dark 

 grey colour, clearly indicating the stratified nature of the deposits. 

 The microscope shows the usual triclinic felspars and green pyroxene, 

 the felspar crystals and fragments being especially abundant, together 

 with fine volcanic dust and glass fragments. There are also very 

 slender needles of a highly refracting yellow-brown mineral, which 

 has much of the appearance of rutile — a mineral scarcely to be 

 expected in such a connexion. 



Falcon Island came into existence owing to a volcanic eruption 

 which occurred in the year 1885.^ The main mass of the island 

 consists of a fine grey ash, mostly quite incoherent, but containing 

 crumbling lumps. The microscope shows this to be a volcanic dust 

 similar to that so widely dispersed during the eruption of Krakatau. 

 It is composed largely of comminuted crystals, among which are 

 recognized felspars, some with twin-lamellation, green augite, and 



1 A Visit to the newly emerged Falcon Island, Tonga Group, S. Pacific, by 

 J. J. Lister, Proceedings of the Eoyal Geographical Society, March, 1890. 



