THE ERUPTION OF BAXDAT-SAW 



151 



rock of Öbandai, has boon made in the laboratory of the Geological Survey Depart- 

 ment. Somehow or other, the amount of foreign ingredients is so large that wo 

 cannot tell from it the true nature of its composition. 



Si 0, 6126 % 



Al. 3 1955 



Fe.,0 3 3-3<; 



FeO 4-06 



CaO 8-20 



MgO 2-54 



Na.>0 3-42 



K 2 1-22 



H,0 1-77 



100-38 



.4 ugite. — This mineral is found in well-defined forms or sometimes in grains. 

 The common type of a twin witli the face gcPx> as a twinning face, is frequently 

 met with. It is usually greenish-yellow in colour, showing a feeble dichroism. It 

 is usually fresh, with glass enclosures, and occasionally small needles of apatite. 



Hypersthene. — The occurrence of the rhombic form of Pyroxene has received 

 considerable attention in recent years. This mineral winch lias hitherto been re- 

 garded as rare, was found to be of wider occurrence than we had expected. v 



The rhombic pyroxene generally appears in more slender sections than the 

 augite ; breadth to length being as 1:3 to 1:5. Under polarized light the inter- 

 ference-colour is weak, and always shows a straight extinction. The pleochroism 

 is very marked; ||' y = light green, JL y = greenish brown. 



On examining the macropinacoidal section, under a convergent polarized ray, 

 we can often very distinctly observe a biaxial interference-figure, which, however, 

 on account of the thinness of the section appears elliptical. The dark cross which 

 traverses the middle of the figure under crossed niçois, passes into an hyperbola as 

 we rotate the section. 



The rhombic pyroxene is often found in a cross shaped twin, the vertical axes 

 of the two individuals making an angle of 00° with each other. This twin is there- 



* An interesting Japanese occuri'enee of this mineral is in the Bonin Islands. Sei' the 

 preceding article in this Volume. — Y. 7v. 



