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 



devel6ped 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 



ibrous natrolite with the cube-like crystals of chabazite occupying 



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



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



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



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



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



: ubmarine eruptions which shattered into fragments the extensive 



)alagonite crusts of flows of basaltic lava. In Chapter XXIV. 



have attempted to show how palagonite is formed on a large 



: cale in the case of such submarine displays of volcanic activity. 



( '.HOCOLATE-COLOURED FORAMINIFEROUS PaLAGONITE-MARLS 



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

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

 i -agments of minerals and a little fine detritus of semi-vitreous basic 

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

 I ut the fragmental nature appears at once in the slide. This is 

 i specially the case with a rock exposed in a stream-course near 

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



