i82 A NATURALIST IN THE PACIFIC chap. 



pyroxene (sp. gr. 272). It has a micro-felsitic groundmass 

 and is referred to the fourth order of the hypersthene-augite- 

 andesites described on page 291. 



About two-thirds of the way up the western slope of the range, 

 there is exposed a coarse palagonite-tuff, evidently an incrusting 

 deposit. Stout crystals of augite can be picked out of it, and it 

 contains also lapilli up to an inch in size of a basic vesicular semi- 

 vitreous basalt. 



Descending the eastern slopes one observes between 1,200 and 

 1,000 feet large blocks of the same grey hypersthene-augite- 

 andesite above mentioned and of a grey granitoid rock of the 

 gabbro type. This last is a hypersthene-gabbro with specific 

 gravity of 275, and belongs to the group of plutonic rocks 

 described on page 250. Its pyroxene phenocrysts are often 

 represented by fibrous bastite. One can scarcely doubt that this 

 gabbro is the plutonic equivalent of the prevailing grey pyroxene- 

 andesites. 



Lower down the slope only small fragments of rocks were 

 exposed, probably derived from an agglomerate. One of the 

 specimens here obtained is a doleritic basaltic andesite (sp. gr. 

 277). Another is a very interesting rock displaying large por- 

 phyritic crystals of a mineral like bronzite in a groundmass 

 originally to a large degree vitreous ; but the glass is now replaced 

 by viridite and secondary crystalline silica. The " bronzite " is the 

 result of the conversion of associated rhombic and monoclinic 

 pyroxene into fibrous bastite. 



From the results of the traverse across this part of the Valanga 

 Range it may be inferred that more or less altered grey basic 

 andesites passing into gabbros chiefly compose it. No doubt at 

 one time it was largely covered with basic tuffs and agglomerates, 

 but these deposits have been almost completely stripped off by the 

 denuding agencies, and were only noticed in one place on the 

 western flank. 



That the northern part of the range towards the Mariko ridge 

 has a similar structure is shown by the character of the loose 

 blocks in the upper course of the Vunimbua River, which takes its 

 rise on these slopes. Amongst those in the river above the village 

 I noticed a solitary block of a coarsely crystalline diorite con- 

 taining prisms of brown hornblende a centimetre in length. 1 But 

 the rocks most frequently represented were propylitic grey hypers- 

 thene-andesites, in which the pyroxene is mostly changed into 



1 This rock is described on p. 251. 



