308 W. F. SMEETH. 



The Felspars are clear and fresh, but are in parts much corroded 

 and contain great numbers of cavities filled with glass. Some of 

 these take the shape of the host bat the majority are irregular in 

 shape, generally connected with one another and with the external 

 glass. These latter doubtless owe, at least their present form to 

 to secondary action. They frequently contain rounded or ring- 

 shaped bodies of an isotropic material which varies in colour from 

 pale green to dark brownish-green, and in places become opaque 

 from the presence of a dark brown or black powder (probably 

 magnetite). They are all doubtless secondary secretions of iron- 

 silicates introduced along cracks and planes of solution in the 

 felspar. The monoclinic felspar is sanidine, which occurs in large 

 single grains, generally simply twinned. The triclinic felspar is 

 much less in" amount, in small grains two or three of which are 

 sometimes aggregated together. The lamellar twinning is well 

 marked and the extinction angles agree closely with those of an 

 albite. 



The Hypersthene occurs in small distinct grains of prismatic 

 habit which usually do not exceed 1 mm. in length. No trace of 

 any other ferro-magnesian mineral is to be observed. The grains 

 though considerably corroded do not appear to have yielded any 

 secondary alteration products (save possibly a little magnetite). 

 In very thin sections they appear to be made up of an aggregation 

 of minute rods parallel to the vertical axis. In thicker sections 

 only a somewhat fibrous arrangement is visible in parts, but there 

 is a strong cleavage in the same direction and a rough cracking 

 at right angles. The grains are distinctly pleochroic, and the 

 following scheme shows their relation to the type mineral. 



Pleochroism : a = reddish-yellow, b = yellow, c = green, principal 

 cleavage 010 which is also the optic-axial plane. a = « and is the 

 acute bisectrix, the optical sign is therefore negative. 



The metallic inclusions so characteristic of some hypersthene 

 are not present, but there are comparatively large crystals of 

 magnetite included as well as numerous small zircons and a few 

 apatite prisms. In addition to these there are a number of 



