338 A NATURALIST IN THE PACIFIC chap, 



areas The hydro-chemical modifications determining the decom- 

 position of these fragments into palagonite, and at the same time 

 the formation of zeolites, have likewise resulted in the complete 

 transformation of these lapilli into ferruginous argillaceous matter 

 (p. 309). The authors, however, of this report do not attribute the 

 frequent occurrence of fragments of basic glass on the bottom of 

 the ocean to the buoyant powers of basic pumice. Unfortunately, 

 the problem does not permit of such a simple solution. Basic 

 volcanic glass, writes Prof Renard, though known only from a few 

 geological formations and from a few eruptions of recent volcanoes 

 at the surface of the continents, appears in abundance and in most 

 typical form among the products of submarine eruptions, as if the 

 deep oceans had been in some way specially favourable to the 

 development of this lithological type (p. 299). 



The palagonite-tuffs of this island are described in detail in 

 Chapter XXIII., and a few general remarks are alone needed here. 

 This altered glass enters into the composition, to a greater or less 

 extent and in varying stages of disintegration, of nearly all the 

 submarine basic tuffs and clays. In the volcanic muds, however, 

 and in the tuffs of mixed character, which are the prevailing 

 deposits, it is associated with other components. Here the question 

 of the origin of palagonite within the deposit does not as a rule 

 arise, since there is nothing to indicate that this material was not 

 derived from rocks previously palagonitised, and the point of main 

 interest is connected with the last stages in the degradation of this 

 substance. There are not a few cases, however, where, unless we 

 assume that the lapilli of vesicular basic glass were ejected in the 

 palagonitic condition from a volcanic vent, we must apparently 

 regard the alteration as having occurred in the tuff. But even this 

 will prove to be by no means a necessary consequence if it can be 

 shown, as I have attempted to do below, that the palagonitic 

 condition exists potentially in a particular type of basic glass and 

 that the effect of hydration is not so much to produce but to make 

 evident a condition that was previously latent. 



It will be therefore of interest to determine whether palagonite 

 occurs in this island independently of the tuff-deposits, and under 

 such circumstances that it may be regarded as having been 

 produced within the rock-mass. An example is afforded in the 

 case of a basaltic flow near Soni-soni Island, which is fully described 

 on page 92. Whilst the lower part of this flow is composed of a 

 hemicrystalline basalt with scanty olivine, the upper portion is 

 made of a basaltic glass which has been broken up or crushed " in 



