is'.i2.] Geology and Paleontology. 416 



Geology of the Tonga Islands. — In the May number of the 

 Geological Magazine, 1891, Mr. Alfred Harker makes the following 

 statements concerning the Ton pi Llands in the South Pacific Ocean : 



" It is well known that most of the Pacific islands which have been 

 explored seem to be built largely of either volcanic or calcareous 

 formations, usually supposed to be of recent origin. Indeed, the idea 

 seems to have been entertained in some quarters that such was the 

 universal construction of the islands. Drasche, writing in b s 7!>. 

 restricted this theory to those islands lying eastward of a certain line, 

 drawn from Kamschatka through .Japan, the Philippines New Guinea, 

 New < 'aledonia. New /• aland. Auckland, and Mae.|uarie I.-lands to the 

 Antarctic Victoria. Even at that time, however, such rocks as clay- 

 slates, graywackes, 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 sug- 

 gested that in many other islands such rocks may be only masked 

 by a comparatively thin covering of organic or volcanic accumulations. 



"It may be inquired, 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. Ena, the most Southerly of the 

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

 ture, and from the Eastern shore of this island Mr. Lister collected a 

 boulder, one of many seen there, which is neither a volcanic nor an 

 organic rock. I have described it (Geol. Mag., April, p. 172) as a 

 uralitized gabbro, and, though some petrologists would prefer to name 

 it diabase, it is unlike any superficially 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 frag- 

 ments of red garnet and blue tourmeline in the calcareous andesitic 

 sandstones largely developed on 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 Falcon Island rocks, all those examined 

 from the Tonga Island appear to be of submarine formation. The 

 absence or presence indifferent strata of any sensible proportion of 

 calcareous matter and organic remains is perhaps related to the more 

 or less rapid rate of accumulation at different epochs of eruption. The 

 volcanic material ejected seems to have been almost exclusively of frag- 



