450 A CONTRIBUTION TO THE GEOLOGY OF VITI LEVU, FIJI, 



the ridges, as such, are the normal result of atmospheric 

 weathering. Whether this is so or not I believe the very 

 markedly regular trend of the Navua, Waidina and Waimanu, 

 together with that of the Waiqa and Wainimala, to be described 

 later, points to a very marked direction of weakness along a 

 W.S.W.-E.N.E. line. 



It is also conceivable that the topography of the Wainivalau 

 basin may be explained in a different manner. The stream might 

 have worked in a general S.S.E. direction across an elevated 

 plateau. Striking the resistant rocks which now constitute the 

 Medrausucu Range, its vertical corrasion might have been 

 checked there, and have proceeded so slowly that its higher 

 waters working constantly over their available area, kept it in 

 the condition of a slowly sinking peneplain, the rate of sinking 

 keeping pace with the corrasion of the range. It would thus 

 have the character of a superimposed river. 



Such an explanation fails, however, to explain the essential 

 geological features, and it is impossible to understand why the 

 stronger Waidina, perhaps fortified by the Upper Navua, should 

 not have excavated an even more extensive plain. 



The evidence then is strongly in favour of heavy faulting, with 

 building up of a very recent volcanic range at the fault-plane. 



Summary of Section i v. — Hydrographic evidence 

 points to the Wainivalau having ver}^ recentlj^ been enlarged at 

 the expense of some of its neighbours. Geological evidence 

 points conclusively to the existence of a heavy fault under the 

 Medrausucu Range, whose existence has been suggested before 

 (p,446) for topographical reasons. Possible evidence is adduced 

 to show that the hornblende andesites of Korobasabasaga are 

 older than the hypersthene andesites of Nabukelevu. A possible 

 alternative to faulting, as an explanation of the topographical 

 features of this part of the island, is discussed and rejected. 



V. — Waiqa Valley. 



Separated from the Rewa Plateau by the northern part of the 

 Medrausucu Range, and from the Wainivalau Valley by the 



