Speight. — Petrological Notes on Eocks from the Kermadecs. '2i^l 



1,000 fathoms of water — in fact, by a greater depth than separates Fiji 

 from Tonga. 



Now, this island is looked upon as an integral part of Fiji, and not popu- 

 lated by haphazard means. If this is the case, there has been either a local 

 sinking of a block of the crust between Kanda vu and Viti Levu , or there has 

 been a general depression .of the whole area. If Kandavu were united by 

 land to Fiji, and severed as the result of such a general sinking, then it is 

 highly likely that Tonga also was connected with a land bridge by a some- 

 what circuitous route in a southerly direction, following a submerged ridge 

 which runs nearly to the Kermadecs, on which the water does not exceed 1,000 

 fathoms. Granting such an elevation, a deep, narrow gulf would extend to- 

 wards the south between Fiji and Tonga. However, arguments resting purely 

 on ocean deeps are very unrehable. as local upwarpings and downwarpings, 

 as well as settling of earth-blocks, are known to occur of such a magnitude 

 as would account for all the facts without any general alteration of large 

 areas of the earth's crust ; and we have the case of Krakatoa, showing 

 how an isolated island near a coast-line may be peopled rapidly by winds, 

 ocean-currents, and birds. 



Tonga. 



In 1891, in a paper on the Tonga Islands read before the Geological 

 Society of London, Lister mentions the occurrence of fragments of garnet, 

 tourmaline, and urahtic gabbro in tuffs of the Eua Island, toward the 

 southern end of the group. No plutonic rocks were found in situ, but the 

 occurrence of garnet and tourmaline suggests the close proximity of an 

 area of metamorphic rocks. In his criticism of that paper, J. W. Gregory 

 mentions the occurrence in the Marquesas Islands of granites and gneisses, 

 and also the fact that these islands have a biological connection with both 

 Malaysia and Chili. I cannot find any authority in the literature at my 

 disposal for Gregory's statement, except that of Marcou, mentioned in 

 Wallace's " Island Life." 



In Jensen's paper on the geology of Samoa* reference is made to the 

 Tonga Islands. This author concludes that they lie on a fold of the earth's 

 crust, with a fault running parallel to them on the western side : " The 

 islands are probably situated over continental rocks on an old shore-line " ; 

 and " that the ridge is probably a structural feature and not the effect of 

 \'ulcanism." The eastern shore of the area would be determined by the 

 Tongan trough, which lies immediately to the east of the islands. 



If we consider the position of the Kermadecs with regard to Tonga, and 

 note the similarity in situation, just to the west of profound ocean troughs 

 in direct aUgnment, and note also the connecting-ridge between the two 

 groups as revealed by soundings, then the probability of a similar origin 

 for both groups should impress us strongly. The Kermadecs would lie on 

 the south-westerly extension of the old Tongan shore-Une, and the volcanic 

 action in the southern group would be only a counterpart of that in the 

 northern group — that is, it would be only a secondary phenomenon re- 

 sulting from great earth-movements in the region. 



Samoa. 

 These islands, according to Jensen, are entirely of volcanic origin. 



* Proc. Linn. Soc. N.S.W., vol. xxxi, 1906. 



