BY H. I. JENSEN. 597 



resorbed at the edges. It appears as if the magma commenced 

 to crystallise at such depths that the temperature and pressure 

 conditions necessitated the separation of magnetite. On rising to 

 the surface the diminished pressure allowed the chemical affinity 

 of the excess of Na 2 for iron oxides to assert itself, hence the 

 magnetite was strongly attacked and segirine formed instead. 



Shortly after magnetite commenced to form, the phenocrysts 

 of. felspar and segirine-augite commenced to separate, and they 

 continued to grow long after the magnetite had finished crystal- 

 lising out. Felspar probably commenced to separate so early 

 because of mass-action ; the magma consisting essentially of the 

 constituents of felspar, it contained throughout an abundant 

 food-supply for nuclei of this substance; for this reason, too, the 

 felspar crystals grew rapidly. The felspar originally consisted 

 of a soda-rich orthoclase, containing about equal molecular 

 proportions of soda and potash. This mineral is stable under 

 high pressure such as obtains at great depths, but on diminution 

 of pressure it tends to split up into soda-orthoclase and albite. 

 As the magma rose to higher levels this change took place in 

 the felspars, and the diminution in pressure further caused a 

 pause in crystallisation during wliich the phenocrysts tended to 

 become corroded and resorbed, especially on the edges. In this 

 way arose the break between the period of the formation of 

 phenocrysts and that of the consolidation of the base. This 

 break is generally, but not always, well marked, the rocks being 

 occasionally serial-porphyritic. The change which the felspars 

 underwent on rising to higher levels led to the formation of 

 microcline microperthite, microcline cryptoperthite, moiree micro- 

 cline, and anorthoclase (in which multiple twinning is only hinted 

 at by the shadowy extinction). These soda-felspars are almost 

 universally dominant in the generation of phenocrysts. 



The formation of aegirine and segirine-augite phenocrysts 

 commenced early, shortly after the commencement of the separa- 

 tion of felspar. They grew less rapidly than the felspar. The 

 presence of nuclei of common augite in the magma, or rubbing 

 against augite rocks in the walls of the magmatic reservoir 

 45 



