Yol. 56.] ERUPTIVE ROCKS EROM NEW ZEALAND. 505 



or geyserite. (See PL XXVII, fig. 6.) Between crossed nicols 

 the hyalite shows the usual double refraction due to tension, broken 

 by an irregular series of dark brushes (parts of dark crosses). The 

 contrast between the hyalite and the isotropic opal-silica is very 

 marked. 



H 43 . Eotorua. — A brecciated rock resembling H 42 , just de- 

 scribed, but apparently more altered by solfataric action. 



Under the microscope, the fragments which give the brecciated 

 character to this rock are seen to be almost wholly pumice; still there 

 are a few fragments in the section as to the nature of which there 

 must be considerable doubt. Probably they are lapilli of rhyolite, 

 very much honeycombed by the hydrothermal action to which they 

 have been subjected. The cementing material is amorphous opal- 

 silica. The fragments do not possess the broad hyalitic borders which 

 surround the fragments in the rock previously described (H 42 ). The 

 lapilli in the breccia, other than those of pumice, have evidently 

 undergone considerable alteration, especially as regards their included 

 crystals of felspar. The total percentage of silica present in this 

 rock, after drying the powder at 100° C, has been estimated by 

 Mr. Philip Holland as 87'89. 



H 44 . Rotorua. — A pink granular rock, easily rubbed to a very 

 fine powder which feels harsh and gritty between the fingers. 



Under the microscope, the section appears to consist mainly of 

 extremely fine pumice-dust and amorphous silica, rendered porphy- 

 ritic in aspect by lapilli of perlitic obsidian and pumice. The 

 section is flecked by numerous irregular dots, patches, and sporadic 

 irregular veins of a substance which in transmitted light seems 

 nearly opaque and of a dark reddish-brown. When examined in 

 reflected light, it is seen to be bright vermilion. The colour is 

 more suggestive of cinnabar than haematite, but, when the substance 

 is heated in a closed tube with dry carbonate of soda, no satisfactory 

 evidence of the presence of mercury is to be detected. The sus- 

 picion that mercury might have been present was increased by the 

 circumstance that cinnabar is sometimes found in connection with 

 hot springs as a result of solfataric action, 1 and that its occurrence 

 in New Zealand has already been noted. 



The total percentage of silica in this rock, when powdered and 

 dried at 100° C, was found by Mr. Holland to amount to 77*79. He 

 also examined it for mercury, but failed to detect any ; he found, 

 however, 0*07 per cent, of organic matter. 2 



1 Dana's ' System of Mineralogy ' 6th ed. (1892) p. 67. 



2 Mr. Holland informs me that he treated a weighed portion of this rock 

 with re-distilled and dry chloroform, and the filtered extract was colourless. 

 The extract, on filtration and distillation, left a slight waxy residue which, 

 heated over a flame on platinum-foil, gave off a resinous odour. The amount 

 of this waxy matter was not more than 007 per cent., as stated above. 

 Mr. Holland noted a similar odour on heating a paraffinoid substance, which 

 he extracted some years ago by means of chloroform from black Scotch peatr 

 He states that the pink colour of this sinter is due to ferric oxide. 



