332 A NATURALIST IN THE PACIFIC chap. 



site of coarser texture but in other respects similar, the felspar- 

 lathes being about "i mm. in length, whilst there is a little micro- 

 porphyritic plagioclase ; (c) a semi-vitreous basaltic rock showing 

 small porphyritic crystals of plagioclase and augite in a ground- 

 mass usually black and opaque, but sometimes smoky and displaying 

 felspar microliths ; (d) a vacuolar basic glass more or less pala- 

 gonitised. 



E. As a specimen of the calcareous tuffs those exposed on the 

 south slope of the Korotini Range at an elevation of 1,850 feet 

 may be given. They contain about 1 1 per cent, of carbonate of 

 lime and inclose a few tests of foraminifera i to 2 mm. in diameter. 

 The other constituents are fragments of semi-vitreous basic rocks 

 and of palagonite, together with fragments of plagioclase and 

 pyroxene crystals and of an amorphous siliceous mineral which 

 behaves optically like chalcedonic silica. When the rock is gently 

 rubbed down, minute fragments of this white mineral can be 

 picked out. They have a wrinkled surface and an irregular form 

 and are not affected by acids. In polarised light they display a 

 rude mosaic or an imperfect radiate structure. 



F. As specimens of the fossihferous agglomerate-tuffs composed 

 mainly of palagonite, those exposed on the high mountain slopes 

 of the Korotini Range at heights of 2,000 feet may be here cited. 

 They are described on p. 154. 



Noie. — The examples of mixed tuffs above given represent 

 only some of the principal types of these deposits. Short descrip- 

 tions of others will be found in the detailed account of the geology 

 of the island. 



Altered Basic Tuffs of Mixed Composition 



These form a group of hard compact rocks, the fragmental 

 character of which is not always apparent in hand-specimens, 

 microscopical examination of thin sections being usually required 

 for the determination of their true nature. They are commonly 

 exposed on the southern flanks of the Korotini Range at the back 

 of Vatu-kawa and Nukumbolo. They are composed of compacted 

 fragments, varying in different localities from i to 5 or 6 mm. in 

 size, of a variety of semi-vitreous basic rocks, the matrix being 

 scanty but often containing zeolites and secondary silica, whilst 

 occasionally secondary calcite is developed. They contain no 



