xii VUINANDI GAP 175 



THE VUINANDI GAP 



I have given this name to the break between the Thambeyu 

 (Mount Thurston) and Koro-mbasanga ranges, where the level of 

 the mountainous backbone of the island descends to about 1,200 

 feet above the sea. This is the route taken by the track from 

 Vuinandi on the shores of Natewa Bay across the island to 

 Lambasa. 



At Vuinandi the mountains recede from the coast leaving a 

 broad level plain extending about two miles inland to the village 

 of Tarawau without rising over 60 feet above the sea. Basaltic 

 rocks are exposed in the spurs that descend from the mountains 

 to the coast on each side of the plain. After traversing the low- 

 lying region that lies between Vuinandi and the main range, one 

 finds on ascending the eastern slopes, en route to Lambasa, 

 basaltic andesites of the usual type prevailing up to 1,000 feet. 

 The upper portion of the dividing range, 1,000 to 1,200 feet, is 

 composed of a more compact basaltic andesite which is often 

 rubbly and in this condition is penetrated by fine cracks, -J- of an 

 inch broad, filled with chalcedony. This rock, which has a specific 

 gravity of 2-85, has a very fresh-looking appearance in the slide, 

 and the segregation of silica does not therefore appear to arise 

 from an alterative change. The felspar-lathes, which are in flow- 

 arrangement, average *n mm. in length, and there is a little 

 residual glass. 



The mountains rise on either side of the Vuinandi Gap to 

 about 2,000 feet. Descending on the west side of the range one 

 follows a stream-course down to a level of 400 feet above the sea, 

 agglomerates and coarse basic tuffs being exposed on the way. 

 The rocks forming the agglomerates are for the most part to be 

 referred to genus I of the hypersthene-augite andesites. They 

 are sometimes compact and sometimes amygdaloidal, the amyg- 

 dules being formed of chalcedony and other minerals, whilst the 

 glass of the groundmass is often altered. 



The track then lay across a spur, 800 feet in height, princi- 

 pally composed of a greyish porphyrite, exhibiting large opaque 

 crystals of plagioclase, 4 to 7 mm. long, in an almost holo-crystal- 

 line groundmass formed of stout lamellar felspars with large augite 

 granules. It is described on page 268 under the porphyritic sub- 

 genus of genus 2 of the augite-andesites, and is an unusual type 

 of rock for this island. After this I descended into the picturesque 



