TUE GEOLOGY OF THE TONGA ISLANDS. b\)0 



Toiiiran volcanoes are members at right angles. It is signifieanfc 

 that Savaii is the largest of either series, and is situated at tlie point 

 where they intersect. Savaii ends the series in a northerly direction. 



The soundings which have been taken in this part of the racific 

 show that the Tonga Islands are situated on a ridge, which rises to 

 within lOOO fathoms of the surface and is prolonged in a south- 

 westerly direction for half the distance between this group and New 

 Zealand. To the south of the Tonga Islands the top of the ridge 

 reaches the surface of the sea, or comes close to it, at the Minerva 

 llcef and three shoal patches indicated in the chart. 



On the eastern side, where soundings have been taken, the 

 slope descends to depths of over 2000 fathoms, and at two points, 

 one to the north-east of Yavau, the other to the east of the southern 

 extension of the ridge, the very great depths of 4530 and 4428 

 fathoms have been sounded. 



On the west the slope appears to be less steep, but a depression 

 (with a d(ipth where it has been measured of over 1500 fathoms) lies 

 between this ridge and the Fiji group. 



The Kermadec Islands, lying between the southern part of the 

 ridge and jSiew Zealand, are the highest points of a large area which 

 is also within the 1000-fathom line. It is, however, separated 

 from the Tonga ridge to the north and New Zealand to the south 

 by depths of over 1500 fathoms. 



Eoth elevations are traversed by a line of volcanoes, which, as 

 Mr. S. Percy Smith * has pointed out, continues the direction of 

 that of the Taupo Zone of New Zealand. Mr. Smith suggests that 

 the volcanoes are situated in the course of a great fissure which 

 extends from Kuapehu, at the southern end of the Taupo Zone, 

 through the Kermadec and Tonga groups to Samoa. This view 

 receives some support from the fact that the eruption of Palcon 

 Island, beginning in Oct. 1885, was contemporaneous with the 

 increased activity (in November) of the geyser ot the white terrace of 

 Lake Ilotomahana, which preceded the great explosion of Tarawera 

 in June of the following year. 



[In the map facing this page the dotted areas between !N"ew Zealand and 

 the Tonga Islands indicate depths of less than 1000 fathoms. (The eastern 

 limit of the Kermadec ai-ea is undetermined.) 



The thick circles indicate active or extinct volcanoes, the broken circles 

 islands whose volcanic nature is doubtful.] 



(b) Islands formed of Sedimentary Volcanic Material. 



Belonging to this division are several islands in the Hapai Group, 

 and the island of Eua. 



The Nomuka Group, which forms the southern division of the 

 Hapai Islands, consists of a number of small islands which are the 

 highest points of an extensive phiteau. Over a large area the top 

 of this plateau is within 50 fathoms of the surface of the sea. 



* 'Geological Notes on the Kermadec Group,' Trans. N. Z.Inst, vol. xx. 

 p. 333. 



a J. G. S. No. 188. 2 T 



