JoLY — Notes on Vokanlc Ash from Krakatoa. 293 



were thickly covered with, this ash around the scene of the erup- 

 tion, and these miracles of structural perfection doubtless abundant 

 in every grain of it. 



Dealing with the constituent minerals in the order of their 

 abundance, the Feldspars first claim attention. They will be at 

 once identified by their colourless transparency ; specimens ex- 

 hibiting a milky decomposed appearance, or showing diffused iron 

 stains of brown-yellow tint being rare. In the latter case, also, a 

 few moments' immersion in hydrofluoric acid generally shows the 

 colour to be confined to the covering of pumice glass adhering to 

 the surface of the crystal. 



This covering of pumice, most noticeable in the feldspars, but 

 also overspreading pyroxene crystals, is obviously the remains of a 

 former matrix of highly vesicular glass ; being occasionally even 

 recognisable as the fragments of bubble walls ; sometimes project- 

 ing along the edges of the crystal, giving a fringed appearance, or, 

 again, overspreading the more developed faces forming a hexagonal 

 pattern often of great beauty and regularity. The appearance, 

 indeed, suggests at first sight such markings, cohesion figures, as 

 are formed when two flat surfaces compressing a viscous material 

 are suddenly separated. It will be found, however, that crystals 

 removed from the block pumice carry with them just such remains 

 of the matrix. 



Optical examination could not safely be undertaken till this 

 coating had been removed by hydrofluoric acid. This operation 

 demands care, as the feldspars themselves are soon decomposed by 

 the acid. 



Two principal forms of feldspar crystals occur ; a columnar and 

 a tabular, the latter most abundantly. 



That the tabular variety is in general plagioclastic is certain. 

 Polysynthetic lines of twinning are frequently observed with 

 polarized light, sometimes crossing at right aDgles. More gene- 

 rally these lines traverse the crystal longitudinally. Carlsbad 

 twinning is common, extinction occuring at some 8 or 9 degrees at 

 either side of the medial line. This form of twinning is seldom 

 absent in crystals of the columnar variety. If such tabular crys- 

 tals as exhibit this medial twinning line be considered as of the 

 same nature as the columnar crystals, a plagioclastic character 

 might be ascribed to the latter, as just such tabular crystals will be 



