BY W. G. WOOLNOUGH. 455 



differences in character of the lower Navua and Rewa valle}^^ is 

 explained as due to the difference in hardness of the rocks to he 

 excavated. The incongruity of grade of the Navua and its 

 tributaries, giving rise to hanging valleys, is described. 



vii. — Wainim ala Valley. 



The upper Wainimala Valley is one of the most interesting 

 and geologically important districts of the island. Up to its 

 junction with the Waisomo* above Vatuvula, and probably for 

 some distance beyond this, it belongs structurally to the Rewa 

 Plateau. Above its junction with the Wailoa to its head, it 

 shows quite a different structure; its banks in this part afford 

 some of the most valuable sections observed at all. 



I chose ISTarokorokoyawa as my headquarters because it lay in 

 the centre of an area of granitic rocks observed by me on my first 

 expedition. I spent a considerable time in trying to determine 

 the relationships of the rock-masses in this region, with very 

 limited success. The jungles here are verj^ dense, and the 

 country very mountainous; exposures are very few away from 

 the rivers, and the problem cannot be considered finally settled. 



The plutonic rocks are of two quite distinct types. The first, 

 represented by the Narokorokoyawa granite,! is a coarse-grained 

 biotite granite, without hornblende, and showing extensive 

 cataclastic structures. The other is represented by the Nadrano- 

 kula quartz-diorite.l It is finer in grain than the granite, has 

 hornblende predominant over biotite, and has suffered less than 

 the granite from shattering, possibly because of its lower quartz 

 content. 



Associated with, and separating these plutonic rocks, is a band 

 of slaty rock. II In my former paper I described a type of this 



* This is not the Waisomo referred to under Sections i. and iv., but a larger 

 stream flowing into the Wainimala from the north. There are at least three 

 Waisomos in the Rewa System, the third coming from Navunitorilau, and 

 entering the Wainimala at Nasava. 



t Former paper, p. 501. X Former paper, p.506. 



II Former paper, p. 514. 



