180 A NATURALIST IN THE PACIFIC ch. xii 



I also rounded the end of the range where it reaches the coast 

 between Lambasa and the valley of Mbuthai-sau. This last 

 locality, which is described on page 218, derives especial interest 

 from the circumstance that here the regions of basic and acid 

 rocks meet. The basic rocks that occupy nearly all the sea- 

 border from Naivaka to Lambasa here become mingled with, and 

 finally give place to, the acid rocks which prevail in all the region 

 eastward as far as Undu Point. 



In crossing the range on the way from Lambasa to Ngele- 

 mumu, I noticed as high as 450 feet basic non-calcareous tuffs 

 displaying a concretionary arrangement suggestive of the prox- 

 imity of an intrusive igneous rock. Further up the western slope 

 occur basic agglomerates, whilst at and near the top (700 feet) 

 there lie on the surface large boulders of a dark grey hypersthene- 

 gabbro having a specific gravity of 27 and belonging to the type 

 of plutonic rocks described on page 249. It is very probable 

 that this gabbro forms the axis of the range ; and we have 

 here no doubt one of the oldest of the mountain-ridges in the 

 island. 



