600 THE GEOLOGY OF THE TONGA ISLANDS. 



Among the volcanic fragments Mr. Harkerhas detected red garnet 

 and tourmaline, whose presence indicates the existence of meta- 

 morphic rocks somewhere in the vicinity. 



A similar partly calcareous rock occurs at the northern end of 

 terrace 6, where the volcanic basis is exposed*. 



The uppermost part of the volcanic formation consists of red, 

 yellow, and brown beds of ashes, which are very much decomposed 

 by weathering. Some are nearly horizontal, others a good deal 

 contorted, but they are never, so far as I observed, regularly inclined 

 at an angle, as though they had formed part of a volcanic cone. 



As mentioned above, dykes of intrusive rock project on the 

 eastern shore. They are composed of augite- and hypersthene- 

 andesites. Large rounded boulders of this rock, mixed with boulders 

 of coral, form the shore, and the natives obtain here the hard 

 rounded stones which are used tor pounding the root of the kava 

 plant, in the preparation of the beverage of that name. The dykes 

 penetrate the beds of tuff and form well-marked projectious at the 

 surface ; they do not, however, penetrate the overlying limestone. 



Besides the intrusive rocks there is a crystalline, plutouic rock in 

 this part of the island, of which I obtained a specimen from a 

 boulder on the shore. Mr. Harker describes it as " uralitized 

 gabbro." Although I did not find this rock in situ, I have no 

 doubt that it exists as a natural constituent of the island. It is 

 highly improbable that a boulder should be brought here by arti- 

 ficial means, for there is no possible landing-place for vessels along 

 the whole of this eastern coast, and the only access to it from the 

 anchorage on the western side is by steep hill -paths. A reef about 

 100 yards wide stretches out from the shore, and on the edge of this 

 reef a vessel would strike, supposing one to be wrecked. I think 

 therefore it may be concluded that the rock occurs naturally on the 

 island. 



The volcanic basis of Eua appears to have been produced by an 

 extensive submarine eruption which formed a mound of considerable 

 extent, and into which lava was injected. 



Tho eruption seems to have occurred at some depth, for the or- 

 ganisms found mingled with the volcanic materials are, according 

 to Dr. Murray, pelagic and not shallow-water forms. Subsequent to 

 its formation and the cessation of the volcanic forces, but prior to the 

 deposition of the overlying limestones, the mound was elevated to 

 at least its present height and considerably denuded. The evidence 

 for this denudation is afforded by the elongated shape, and the 

 steepness of the eastern slope of the volcanic basis which underlies 

 the limestone ; these imply the removal of a considerable part of the 

 original mound, which is built up in the main of horizontal layers. 

 After its denudation the island subsided again, before the deposition 

 of the limestone formation. 



The limestones invest nearly the whole of the lower part of 

 Eua up to a level of about 500 feet, and above this level they occur, 

 though less continuously, as high as the summit. 



* Slide No. 1273. 



