254 ^ KOTö. 



the form of isometric grains ; sometimes, liowever, elongated ones 

 (1 mm.) are observed, and along their longer axis rnn a few suture 

 lines which are traces of the characteristic cleavage of aiigite (Fig. 12, 

 d and e). The extinction of light takes place at a very high angle, 

 usually 30° or more, w^ith regard to the trace of the cle;ivage. It is 

 colourless or iliint-green. indicating the paucity of iron in it. The 

 colouring pigment, slight as it ie, is, however, not uniformly distribut- 

 ed, the peri})hery is rather of a lighter shade than the interior. The 

 mineral presents another very characteristic feature which is here 

 worthy of short consideration. There are many short, black streaks, 

 which run strictly parallel to the cleavage- direction, and are C(^ntined 

 exclusively to the interior of the crystal (Fig. 12, d and e). Examin- 

 ed with high powers these interpositions are found to he nothing but 

 negative, long, rectangular sj>aces, tilled witli air, or water with air- 

 bubbles, and the grains consequently look not unlike the diallagic 

 modificfition of augite. 



Recently, Prof. Judd advanced a beautiful theory of the schil- 

 lerization — a phenomenon produced through tlie secondary infiltra- 

 tion of some substance into the spaces formed by certain solvents 

 along the so-called " solution-plane," or the plane of chemical weak- 

 ness, when the rocks containirig feldspar and also augite are sul^ject- 

 ed to high strains at great depth.' In the last number of the 

 Mini'ralotjical Magazine^, Judd has again submitted augites to the 

 same line of treatment which he had so earnestly advocated in his 

 former papers. The orthopinacoid of augites, he says, is the normal 

 solution-plane, and if once the twinning lamellation is induced to de- 

 velop in crystals, then along the planes of chemical weakness just the 

 same thing may be effected as in those of labradorite. I can not hut 



1 Mineralogical Magazine, Vol. VII. (1886). 



2 Ibid. Vol. IX. p. 192. 



