515 



in the gabbroid rocks and pyroxenites, and slightly less in 

 the peridotites, are the dimensions of the essential minerals, 

 half a millimetre being perhaps the average diameter of an 

 accessory mineral grain, ilmenite and some picotites are 

 larger, pleonaste and the rest of the picotites smaller. 

 There are no idiomorphic minerals in the rocks, but an order 

 of crystallisation can be made out by the relative idiomor- 

 phism of the minerals. In anorthosite the pyroxene diallage 

 appears to have formed first, though this is not quite 

 definite, for in the pyroxene there is sometimes a little 

 spinel (for the significance of which see later). In a diallage 

 gabbro composed of about sixty per cent, of felspar and 

 forty per cent, diallage, the latter is most distinctly ophitic 

 enclosing large plates (but not laths) of felspar (see lig. 7). 



: structure in gabbro plagioclase 

 iron ores in the magma appears to affect 

 the order of consolidation. In a rock approximately of the 



