2i8 A NATURALIST IN THE PACIFIC chap. 



these groups of hills into separate islands and would cover much 

 of the inland plains with the sea. The hills attain an elevation 

 of 1,200 or 1,300 feet a mile or two inland and descend often as 

 bold promontories to the coast. I will refer in order to the differ- 

 ent parts of this sea-border. 



(i) The Sea-border between Lambasa and Mbuthai-sau 



In the sea-border between Lambasa and Mbuthai-sau we have 

 the junction of the regions of basic and acid rocks, the former 

 extending westward to Naivaka, the latter reaching to Undu Point. 

 In such a locality the two types of rocks might be expected to be 

 associated, and this is what occurs. Acid pumice-tuffs and basic 

 agglomerates, sometimes associated, are here displayed. In the 

 low hills between Lambasa and Vuni-ika Bay, which lies west 

 of Mbuthai-sau, I found basic agglomerates prevailing, together 

 with some acid pumice-tuffs. The blocks in the agglomerates are 

 composed of a blackish semi-vitreous pyroxene-andesite (sp.gr. 2'68), 

 which is characterised by prismatic pyroxene in the groundmass, 

 and is referred to genus 6 of the second sub-order of the hypers- 

 thene-augite andesites described on page 287. 



East of Vuni-ika on the way to Mbuthai-sau, at an elevation of 

 about 50 feet above the sea, dark tuffs containing small fragments 

 of reef-limestone are exposed in a cutting. A little farther on 

 there is a considerable deposit of a pale grey rhyolitic pumice- 

 tuff, a soft stone easily worked, and indeed now quarried by the 

 plantation authorities. It contains no lime and in microscopical 

 characters is scarcely distinguishable from a sample of fine pumice 

 debris collected by me in the Chirica district of Lipari Island. It 

 is made of fragments up to a centimetre in size, of ordinary 

 fibrillar pumice in a matrix of much finer material of the same 

 nature. Portions of crystals of glassy felspar (oligoclase and 

 sanidine) also occur in it, together with some quartz and rhombic 

 pyroxene. 



The association in this locality of acid and basic eruptive pro- 

 ducts was observed by Dana in 1 840 in the cliffs of " Mali Point." 1 

 It is not quite clear whether Mali Island, which lies immediately 

 adjacent to the coast, is here alluded to, or whether it is a head- 

 land opposite to it. Dana describes the deposits displayed in 

 these cliffs as coarse aggregates of fragments of pumice and 

 decomposing trachyte, which pass on the one side into fine 

 1 Geology of the United States Exploring Expedition. 



