JoLY — Notes on Volcanic Ash from Krahatoa. 



The tabular variety is represented in the photographs by the 

 remaining crystals shown in the photograph (PI. sii. fig. 1), which 

 is taken at an enlargement of about 20 diameters. A very good 

 specimen is shown, magnified 35 diameters, in Plate sii. figure 2 : 

 enclosures, vitreous, containing crystals, will be noticed. Grrowth 

 striations and the lace-like pattern of adhering-glass are also 

 shown. The presence of growth striations or zones, very common 

 in these feldspars is generally assumed to indicate a more or 

 less tranquil formation. 



The Pyroxenes are a noticeable feature in this ash. The 

 rhombic variety is by far the most abundant, indeed the mono- 

 clinic is scarce, and present usually in badly developed or small 

 crystals. A group of the rhombic pyroxenes is shown in Plate 

 xiii. figure 4. Enlargement 15 diameters. 



These crystals are olive-green to brown in colour ; translucent. 

 Their pleochroism is well marked, the colouration following the 

 order assigned to Hypersthene, by MM. Fouque and Levy (Minera- 

 logie Micrographique) : that is, parallel to their principal axis 

 dark-green, parallel to the macrodiagonal, red, with intermediate 

 tints of brown. 



This mineral will be found to extinguish parallel to its pina- 

 coidal faces, not alone when viewed longitudinally, 

 but also when placed with its principal axis in a 

 vertical position between crossed nicols. Treating 

 in this way the nearly central crystal, the one ex- 

 hibiting a pointed termination at each end in the 

 group (PL xiii. fig. 4), the exquisite perfection of 

 its minute form became apparent. 



This crystal, which parallel to its longitudinal 

 axis shows pinacoidal and prismatic faces, the 

 former dominant, shows four principal termi- 

 nating planes, apparently octahedral. Two other 

 faces, brachydomes seemingly, are well defined. 

 Other minute faces are visible, scarcely determin- 

 able. The adjoining woodcut, a sketch of its 

 appearance viewed in a direction inclined a little 

 upwards, and to the left of the brachydiagonal, shows some of its 

 faces. 



The photograph (PI. xiii. fig. 3) conveys but a faint idea of a very 



