252 A. Harher — Rocks from the Tonga Islands. 



fibrous bastite. Magnetite grains and crystals occur sparingly. 

 The most plentiful secondary product is yellow-green pleocliroio 

 epidote, which seems in part to replace some of the larger felspars. 

 The slide shows numerous irregularly shaped patches of andesite 

 differing from their matrix. They are sometimes of finer texture, 

 sometimes coarser, or again have a fine ground enclosing porphyritic 

 felspars. These patches do not share in the general flow structure 

 of the rock, and seem to be relics of a solid crust formed on the 

 surface of a eoulee, and broken up by subsequent movement of 

 the mass. 



Mr. Lister notes three dykes exposed on the eastern shore of Eua. 

 These cut through the volcanic deposits, but are older than the 

 overlying limestones, which they do not penetrate. The specimens 

 are of a dull-grey andesite, sometimes showing porphyritic felspars 

 to about an eighth of an inch long, or little dark spots which 

 represent decomposing pyroxene. 



Under the microscope these rocks show innumerable microlites of 

 felspar imbedded in an isotropic base. When porphyritic felspars 

 occur, they exhibit Carlsbad and albite twinning and strong zonary 

 banding in polarized light. Augite is not recognizable in the 

 ground-mass, but occurs in more or less idioinorphic crystals, 

 colourless or nearly so in section, among the earlier minerals. With 

 it is associated a pale yellow enstatite in good prisms terminated 

 by the dome (102) [slide 1261]. Magnetite is present either in 

 crystals of the earlier consolidation or in little granules in the 

 ground-mass. As secondary products we find calcite from the 

 felspars, delessite and calcite from the augite, and green dichroic 

 bastite from the rhombic pyroxene [1260]. A specimen [1259] 

 from the northerly dyke shows the usual microlitic ground traversed 

 by distinct branching veins which consist mainly of larger, though 

 still mostly untwinned, crystals of felspar, extinguishing nearly 

 parallel to their length, and perhaps referable to sanidine. These 

 veins are probably segregations marking the last phase in the 

 consolidation of the mass. 



The fragmental volcanic rocks of Eua are well bedded in nearly 

 horizontal strata, and exhibit frequent alternations of types difi"ering 

 in degree of coarseness, etc. Some are of the nature of volcanic 

 dust, forming bands of grey to red colour according to their fresher 

 or more decomposed condition. Among the finely divided material 

 of these bands may be recognized little rounded or irregular bits of 

 brown glassy matter, and broken crystals of clear felspar, green 

 augite, and occasionally pale enstatite. The extinction-angles 

 measured on cleavage-flakes of the felspar show that more than one 

 member of the soda-lime series is represented. The same materials 

 are recognized in various red-brown earthy rocks from the higher 

 ground of the island, apparently formed by atmospheric weathering, 

 and in the fine sand from the beds of the rain-channels in the 

 neighbourhood. The fragments in these fine ash-beds are fairly 

 uniform in size, though occasionally a few small lapilli occur. 

 These deposits are mostly free from carbonate of lime, but there 



