A. Harker — Rocks from the Tonga Islands. 251 



the universal construction of the islands. Drasche,^ writing in 1879, 

 restricted this theory to those islands lying eastward of a certain 

 line, drawn from Kamschatka through Japan, the Philippines, New 

 Guinea, New Caledonia, New Zealand, Auckland and Macquarie 

 Islands to the Antarctic Victoria. Even at that time, however, such 

 rocks as clay-slates, greywackes, etc., had been recorded in the 

 Chatham Islands^ and New Britain,'' east of Drasche's line, and 

 leptinites,* granite, and gneiss ^ in the Marquesas, far to the east. 

 Later researches have proved the existence of numerous crystalline 

 rocks, igneous and metamorphic, in the larger islands of the Fiji ^ 

 and Solomon'' archipelagos, and suggested that in many other 

 islands such rocks may be only masked by a comparatively thin 

 covering of organic or volcanic accumulations. 



It may be iuquii'ed, then, whether the Tonga Islands show any 

 indication of the existence of denuded crystalline rocks beneath the 

 newer deposits. No such rocks have been found in place, and the 

 evidence available is very slight. Eua, the most southerly of the 

 larger islands, differs to some extent from the rest in geological 

 structure, and from the eastern shore of this island Mr. Lister 

 collected a boulder, one of many there seen, which is neither a 

 volcanic nor an organic rock. I have described it (Geological 

 Magzine for April, p. 172) as a uralitised gabbro, and, though some 

 petrologists would prefer to name it diabase, it is unlike any super- 

 ficially erupted lava. Further, there is no doubt that it is derived 

 from the island on which it was found. The only other suggestive 

 point is the rare presence of minute fragments of red garnet and 

 blue tourmaline in the calcareous andesitic sandstones largely 

 developed in the same island. These fragments, blown out from 

 a volcano, point to the existence of metamorphic rocks below, 

 though at what depth it would be idle to speculate. 



"With the exception of the dykes mentioned below, the volcanic 

 rocks of Eua seem to consist entirely of fragmental accumulations, 

 no lava-flows being seen. Mr. Lister's collection, however, contains 

 a specimen of a boulder from the same locality as the one mentioned 

 above, which is a grey compact-looking andesite of specific gravity 

 2-618. It shows under the microscope a kind of flow-brecciation 

 which causes us to compare the rock with extruded lavas rather 

 than with the dykes exposed near the locality in question. The 

 rock [1258] consists of very numerous little plagioclase crystals 

 imbedded, with marked parallel arrangement, in an isotropic ground. 

 The felspars are lath-shaped to acicular, and twinned or simple 

 according to their size. There is little or no augite, but an 

 occasional crystal of rhombic pyroxene transformed into pale green 



1 Neu. Jahrb. 1879, p. 265. 



^ See Darwin's "Geological Observations:" cf. Haast, Trans. N.Z. Inst. vol. i. 

 p. 180, 1869, and Hector, ibid. vol. ii. p. 183, 1870. 



3 Meinicke, " Inseln des Stillen Oceans," vol. i. p. 133, 1875. 



* .Tardin, Mem. Soc. Imp. Sci. Nat. Cherbom-g, vol. iv. p. 55, 1856. 



s Marcou, "Explic. Carte Geol. de la Terre," p. 185, 1875 ; authority not cited. 



^ Wichmann, Tscb. Min. Petr. Mitth. vol. v. p. i. 1882. 



'' Guppy, "The Solomon Islands, their Geology," etc., 1887. 



