BY W. G. WOOLNOUGH. 529 



There are moderately numerous amygdules partially or wholly 

 filling small irregular cavities up to 1 mm. across. The secondary 

 material appears to consist mainly of opal, chalcedony, and a 

 serpentinous substance. In many cases the walls of the cavities 

 appear to be coated with a thin layer of perfectly clear and 

 isotropic substance whose refractive index is lower than that of 

 Canada balsam; this is proba'hly opal. Next comes a zone of 

 fibrous material, slightly milky, exhibiting very weak double 

 refraction. The extinction of the fibres is parallel to their length, 

 and the layer has all the appearance of chalcedony. The surface 

 of the opaline layer maybe described as "micromammillary," and 

 the chalcedonic fibres, standing at right angles to this surface, 

 are therefore somewhat radial. The serpentinous material is 

 irregularly distributed. It is greenish-brown in colour and shows 

 very faint pleochroism. The structure is distinctly fibrous radial, 

 and the double refraction is noticeably higher than that of felspar. 

 These decomposition products are also distributed through the 

 rock and fill cracks which pass through all the minerals indis- 

 criminately. It is possible that the minerals described as opal 

 and chalcedony may be zeolites. The serpentinous material is in 

 part derived from the augite, but probably much of it represents 

 the material removed during the destruction of the hornblende. 



Hornblende Andesite (Korobasabasaga). 

 Plate xxxvi., fig. 7. 



As stated in the first part of this paper, no outcrops of rock 

 were met with in the ascent of Korobasabasaga until the crest of 

 the ridge was reached, when the "plug" filling the southernmost 

 summit was encountered. 



The rock consists of an exceedingly coarse breccia. The base 

 consists of almost white comminuted fragments of lava similar 

 to that forming the ejected blocks. The dark prisms of horn- 

 blende are very noticeable constituents of it, as are also felspar 

 splinters. The ejected blocks are very numerous and are mostl}-- 

 fairly angular in shape. 



Macroscojnc character's^. — In hand specimen the rock is very 

 light grey in colour, with very obvious glassy felspars and black 



