464 MESSES. J. PARE AND F. RUTLET ON THE [Aug. 1 899, 



which may have been present having apparently been converted 

 into limonite. A few small felspars give extinction-angles suggestive 

 of anorthite, but they are muck altered. The larger porphyritic 

 felspars are represented by pseudomorphs of chalcedony. Pyrites 

 is present in these rock-fragments in very considerable quantity, 

 forming irregular patches, aggregates of crystals, minute individual 

 crystals, or mere specks. Some small crystals of magnetite are also 

 to be seen. On the whole, the rock appears to be an andesite- 

 tuff, very greatly affected by solfataric action. 



H^Q. "Waihi. — A compact bluish-grey rock, containing numerous 

 small crystals with a vitreous lustre. The specimen shows a cavity 

 about I inch in diameter, lined with mammillated chalcedony and 

 with some snow-white crust. The cut surface takes a high polish. 

 This specimen is stated to come from a ' quartz-blow.' 



Under the microscope the rock does not appear to be clastic, as 

 in H^f,, but to be an altered andesite, which, however, like the 

 preceding, has been highly impregnated with silica. It contains 

 numerous porphyritic crystals of andesine, labradorite, and occa- 

 sionally anorthite. Some of these are honeycombed with irregular 

 inclusions, containing devitrified glass (PI. XXXIII, fig. 8). Brown 

 spherulites are also present, and large brown spherulitic growths 

 sometimes occur as borders around groups of felspar-crystals. More- 

 over, porphyritic quartz showing a rude perlitic structure, similar 

 to that described by Prof. Watts, may be seen in this section. 



Appendix. 



Since fragments of andesite and minerals, in many cases doubtless 

 derived from andesitic tuffs, occur in the rocks which form the 

 subject of this paper, it may be well to add a brief description of a 

 few of these andesites from Waitekauri. Although many New 

 Zealand andesites have been described by Capt. Hutton, those from 

 Waitekauri do not seem to have been specially noticed. 



Hg^ [192]. Waitekauri. — A pale bluish-grey rock, with por- 

 phyritic quartz and numerous blackish-green crystals. 



Under the microscope, the groundmass of the section presents a 

 hypocrystalline character, which, from the presence of microlites, 

 occasionally becomes almost hyalopilitic. This groundmass is 

 plentifully charged with globulites, and contains a few tolerably 

 large, rounded, porphyritic crystals of quartz, one of which gives a 

 good positive uniaxial figure in convergent light. The rock also 

 contains porphyritic crystals of plagioclastic felspars and pseudo- 

 morphs of haematite after hornblende, transverse sections of the 

 latter sometimes giving the prismatic angle of 125°. The felspars 

 are oligoclase and andesine. Small opaque pseudomorphs, appa- 

 rently after mica, are plentiful in the section. Little crystals of 

 zircon and magnetite are also present. The rock appears to contain 

 some tridy mite. It is essentially a hornblende-andesite. 



