v MOUNT SEATURA 63 



The rocks of this ancient volcanic mountain are almost all of 

 the massive basic type, and except at the mouth of the Ndriti Gap 

 hardly ever display a scoriaceous character. It is also noteworthy 

 that no detrital rock, whether agglomerate, tuff, or tuff-clay came 

 under my observation. The rocks exposed on the surface are 

 mostly blackish brown olivine-basalts and porphyritic basaltic 

 andesites, the former much prevailing. In the northern portion, 

 however, grey olivine basalts of a different type occur. In the great 

 crateral hollow, which I have named the Ndriti Gap, are displayed 

 numerous dykes formed of highly altered basaltic rocks that may 

 be classed among the propylites. 



The dense forest that clothes the greater part of this mountain 

 offers many serious hindrances to geological exploration. Except 

 in the northern portion, views of the surroundings are very limited, 

 and one has often to rely mainly on the aneroid and the compass 

 to obtain correct ideas of the contours and general configuration. 

 During most of the time spent in the southern part of the mountain, 

 my work was greatly impeded by heavy rains, and from this cause and 

 from the frequent necessity of following up the stream-courses and 

 of crossing rivers in flood, I was usually wet through all the day. 



(a) The Eastern Slopes of Seatura. — The basaltic flows, of 

 which this mountain is principally composed, are best observed on 

 the eastern side where the original volcanic slopes are preserved. 

 Although the rivers have worn such deep valleys into the mountain 

 sides, it is however not often that any great exposure of rock 

 occurs, on account of the dense forest-growth over much of this 

 region. It is only occasionally that the columnar structure of these 

 old basaltic flows is displayed. It is especially well exhibited in 

 the face of a waterfall, distant about two miles in a straight line 

 from Tembenindio and elevated about 700 feet above the sea. 

 Here there is an exposure to the extent of 25 feet of huge basaltic 

 vertical columns, four to five feet across, and pentagonal in form. 

 The rock is a blackish basalt with scanty olivine and a specific 

 gravity of 2*87. It is referred to genus 25 of the olivine-basalts 

 which is described on page 259. Micro-phenocrysts of plagioclase 

 and a few of augite occur, the olivine being mostly replaced by 

 pseudomorphs. The felspar-lathes of the ground mass average 

 •18 mm. in length, and there is a little brown opaque interstitial 

 glass. Boulders and fragments of a closely similar basalt, with a 

 specific gravity of 2-9, lie about on the surface in this region. The 

 Seatura slopes here abut on the plateau of Na Savu, formed largely 

 of volcanic agglomerates, to be subsequently described. 



