xxiii PALAGONITE-MARLS 



335 



where zeolites and at times secondary calcite have been produced 

 in abundance as a result of the alteration. They present themselves 

 in the mass as mottled grey rocks which when examined in thin 

 sections are seen to be composed in great part of fragments of more 

 or less palagonitised vacuolar basic glass, whilst zeolites are 

 extensively developed in numerous irregular cavities and in the 

 interspaces. Although displaying no organic remains, their sub- 

 marine character is indicated as at Nandua by the circumstance of 

 their occurring as horizontal beds overlaid by pteropod-ooze 

 deposits, or as at Tembe-ni-ndio by their forming part of a series 

 of horizontal beds with a shelly limestone and a foraminiferous 

 palagonite clay overlying them. 



The fragments of bottle green basic glass vary usually between 

 i and 4 millimetres. They were originally vacuolar and at times 

 fibrillar from the lengthening out of the minute steam-pores ; but 

 through the palagonitic change these characters have been often 

 disguised, and it is only at times that the unaltered glass is 

 observed. Plagioclase and sometimes augite and occasionally 

 olivine formed phenocrysts in the original glass. The zeolites, 

 which include chabazite and natrolite, may be so extensively 

 developed that they make up a fourth or a fifth of the rock mass. 

 One may observe them in cavities where the walls are lined by 

 fibrous natrolite with the cube-like crystals of chabazite occupying 

 the interior. The calcite is usually subordinate to the zeolites, but 

 sometimes the tuff contains as much as 10 per cent of this mineral, 

 which is evidently of secondary origin. . . . The history of these 

 tuffs in the district of Nandua and Ulu-i-ndali is no doubt applic- 

 able to these deposits in other localities. They are the products of 

 submarine eruptions which shattered into fragments the extensive 

 palagonite crusts of flows of basaltic lava. In Chapter XXIV. 

 I have attempted to show how palagonite is formed on a large 

 scale in the case of such submarine displays of volcanic activity. 



Chocolate-coloured Foraminiferous Palagonite-marls 



We have here hard, somewhat calcareous, clay-rocks which con- 

 sist in great part (nine-tenths) of fine palagonite debris with some 

 fragments of minerals and a little fine detritus of semi- vitreous basic 

 rocks. Some hand-specimens would be taken for pure palagonite ; 

 but the fragmental nature appears at once in the slide. This is 

 especially the case with a rock exposed in a stream-course near 

 Rewa on the shores of Savu-savu Bay (see page 95). The 



