and their Contact Zones. 41 



rarely magnetite, and have a somewhat rough-looking sur- 

 face. A net- work of cracks traverses them, which are filled 

 by magnetite. Similar irregular cracks radiate from these 

 inclusions and connect one with the other. This mineral 

 polarises in bright shades of red and green, and has some 

 general resemblance to quartz in its chromatic effects. Not 

 one instance afforded me any outline from which the crystallo- 

 graphic system could be deduced. The general appearance, 

 structure, and chromatic polarisation suggested olivine, and 

 this is confirmed by the mineral being readily attacked in 

 the slice by hydrochloric acid. After a little time, when thus 

 tested, particularly if heat is applied, these inclusions become 

 opaque, and remain dark when rotated between crossed 

 nicols. The whole of the magnetite is also removed from 

 the network of cracks. It is remarkable, however, that few, 

 if any, traces of serpen tinisation are apparent. These inclu- 

 sions of olivine somewhat resemble similar ones which I 

 have observed in a sample of gabbro from Volpersdorf. 

 A third inclusion belongs to the spinel group. Scattered 

 irregularly throughout the whole mass, but I think almost 

 wholly in the amphibole, are many grains and masses of 

 grains of irregular outlines, and when of sufficient tenuity 

 having a rich green colour. Very rarely I have observed 

 instances where these grains were rectangular — i.e., in sec- 

 tions of the regular octahedron. When examined in a thin 

 slice ready for mounting by reflected light, these grains are 

 seen to stand above the surrounding mass and to be black 

 and lustrous. As tested by a needle-point, I estimate their 

 hardness as over 7. They are wholly unaffected by hot 

 hydrochloric acid. By transmitted light the grains are, 

 where sufficiently thin, translucent and of a rich green 

 colour. Between crossed nicols, and also examined by the 

 stauroscope, they prove to be isotropic. These physical and 

 optical characters point to one of the spinel group, either 

 picotite or pleonaste. Pyrite occurs sparingly throughout 

 this rock in grains. It is macroscopically visible, and in the 

 thin slices may be identified with those opaque black 

 masses which remain after the magnetite has been removed 

 by hydrochloric acid. 



The alterations which have taken place in this amphibole- 

 anthophyllite commence by the production of a fibrous 

 structure parallel to the axis c. This destroys the glassy 

 aspect of the perfect mineral, but does not alter its mono- 

 clinic optical character. I have observed round the edges 



E 



