XIII THE MARIKO RANGE 185 



fine cracks with decomposition products in the interior of the 

 crystals. ... It is thus seen that in general structure the Na Kula 

 Ridge represents the main axis of the Valanga Range to the north. 



The valley of Na Kula is occupied by a river which does not 

 empty itself, as one would expect, into Savu-savu Bay, but turns 

 oiT sharply to the south at right-angles to its previous course, and 

 after breaking through the coast range, opens into Naindi Bay. 

 This peculiarity has attracted the attention of the natives. The 

 village of Sawa-Ndrondro, which lies about i| miles up this valley, 

 is not elevated more than 50 feet above the sea. The gradient is 

 evidently not only very slight but is also irregular, so that in their 

 upper course about 3 miles inland, where the elevation is only 130 

 feet, the waters of the river are partially checked in their flow and 

 form extensive swamps where the ''vitho" or wild sugar-cane 

 flourishes. 



(5) Concluding Remarks on the Valanga Range.— It 

 may be inferred from the geological structure of the range that it 

 is one of the oldest in the island. The agglomerates and tuffs 

 that enter so largely into the formation of most of the other 

 mountain-ridges are here to a great extent absent, except in the 

 lower flanks ; and we have exposed the axis of the range composed 

 of more or less altered grey pyroxene and hornblende-pyroxene 

 andesites passing, as appears to be the case, into gabbros and 

 diorites. It is true that the exposure of the gabbros is limited and 

 that only a single block of diorite came under my notice ; but 

 this might be looked for where the plutonic rocks are deeply 

 seated. Although far overtopped by the neighbouring agglomerate 

 mountain-ridge of Mariko, the Valanga Range would seem to date 

 back to a much earlier stage in the history of the island. 



The Mountain-Ridge of Mariko 



This mountain-ridge, which trends nearly east and west and 

 joins the Valanga Range, rises in mass to a height of rather over 

 2,000 feet. Above this elevation it terminates in several short 

 conical peaks, of which the highest, 2,890 feet, is named Mariko, the 

 Drayton Peak of the chart. One of the peaks, lying a little to the 

 east of the summit, and apparently between 100 and 200 feet 

 lower, is called the Vatu-mbutho or White Rock. In the profile 

 of the range, as seen nearly " end-on " from the distant south shore 

 of Natewa Bay, it would appear to be rounded in its upper part. 

 Its true outline, however, when viewed in length, is, as described 

 above, namely, a massive ridge with various peaks. 



