498 ME. V. RUTLEY ON [Aug. I9OO,- 



in Auvergne. The section contains numerous colourless micro- 

 lites and a few crystals of sanidine, also some opaque white matter 

 forming small pseudomorphs after a mineral which appears to 

 have crystallized in approximately octahedral forms. In one 

 instance a fragment of an opaque black mineral is surrounded by a 

 thin opaque white crust. This may possibly represent a superficial 

 alteration of ilmenite into leucoxene. 



Small fragments of rock are present, but not plentiful, in this 

 section. They are so much altered that it is unsafe to express any 

 opinion about them other than that they seem, for the most part, to 

 have been derived from rhyolites. 



The great hardness of the rock is to be attributed to the tridymite 

 with which it is so abundantly impregnated. 



H 31 . "VYaihi Beach ('older rhyolite ' of Park). — A pale greyish- 

 white spherulitic rock, containing small dark specks and colour- 

 less crystals with a vitreous lustre, also a few dark micaceous-looking 

 scales. The spherulites range from about g inch in diameter to 

 smaller dimensions. 



Under the microscope, the rock appears to consist almost wholly 

 of spherulites which seem originally to have been of the brown 

 microfelsitic type. They are still brown, but are almost isotropic, 

 in the centres perfectly so, but the peripheral portions usually 

 transmit light very feebly between crossed nicols. 



The spherulites often have polygonal boundaries, as though 

 they had pressed against one another, and among them occur 

 numerous porphyritic crystals and fragments of crystals, frequently 

 showing very distinct evidence of corrosion. These porphyritic 

 crystals may be recognized as quartz, felspars, biotite, and pyrites. 

 The quartz appears to have been cracked and affected by heat. The 

 felspars are all practically isotropic, and can consequently be 

 identified by the forms of their sections alone. Biotite is not 

 very plentiful in this rock : basal sections may be recognized by 

 the approximately hexagonal forms giving angles of 60°, but in 

 convergent polarized light no satisfactory interference-figure can be 

 seen, since the crystals are rendered more or less opaque by partial 

 alteration into limonite. One section transverse to the cleavage 

 shows the characteristic pleochroism and absorption. The pyrites 

 occurs in cubes, groups of cubes, and irregularly-shaped patches. 

 The section is speckled by diminutive opaque crystals, many of 

 which appear to be magnetite or pseudomorphs after that mineral. 



One of the most interesting features of this section is, however, 

 to be found in a small irregular area in which a coarse mosaic of 

 felspar and quartz may be seen between crossed nicols. In con- 

 vergent light the quartz shows partial uniaxial interference-figures, 

 and, in one or two instances, a positive sign. The felspars also 

 show partial interference-figures, a dark, curved brush sweeping 

 diagonally across the field. This area in the section is, therefore, 

 unquestionably f el site, so far as mineral constitution is concerned .. 

 Structurally it differs from an ordinary felsite in the circumstance 



