EUA ISLAND. 181 



flanks of Eua indicate that the island and the plateau from which it rises 

 are a part of the great Tonga Plateau. In fact, it is not separated 

 from Tongatabu by as deep channels as those wliicli divide the Nomuka, 

 the Haapai, or the Vavau plateaus one from the other. The soundings 

 to the south of Tongatabu, in the direction of Pylstaart^ Island, indicate a 

 southern extension of the Tonga Ridge, of nearly ninety miles, with irregu- 

 lar depths (PI. 214). 



Eua was the first of the Tonga Islands we examined. It was survej'ed 

 by Captain Aldrich in the " Egeria " in 1888. From the description given 

 by Dana, which was obtained at second-hand, I imagined that this island 

 would be simply a volcanic ridge, surrounded in places by a very narrow 

 fringing reef. On approaching it from the east, and striking the northern 

 point, we were much surprised to find it composed of elevated coralliferous 

 limestone, rising at points to nearh' 1100 feet. The crest of the eastern 

 ridge, which runs parallel to the eastern coast, is not much more than three 

 fourths of a mile from the shore. Its highest points .are towards the south, 

 whei'e the successive terraces, indicating periods of inaction during the eleva- 

 tion of the island, are clearly seen, — terraces which run from the southern 

 point to the northern extremity, interrupted by high vertical cliffs, the 

 whole extent of the eastern face (Pis. Ill, figs. 2, 3 ; 112, 113). The eastern 

 ridge forms the northern extremity of the island (PL 111, fig. 3); the steep 

 vertical cliffs extend but a short way around the western coast of Eua. 



The highest point of the ridge on the western face, towards the south- 

 ern extremity of the island, is not Tuore than 560 feet; it is cut by a small 

 river coming out at the English Roads (PI. 212, fig. 4), and draining the 

 central valley of Eua. formed by the two ridges which run, the one parallel 

 to the east coast (PI. 114), and the other parallel to the west coast (PI. Ill, 

 fig. 1). On the west coast, however, the slope from the highest summit 

 towards the shore is quite gradual, as is also the western slope of the 

 eastern ridge towards the sink or drainage valley, which occupies the 

 central part of the island. 



The southern slope of the western ridge is .somewhat steeper than that 

 of the eastern ridge ; its summit forms a plateau varying from one fourth 



1 A. Chart 24-21. 



