BY VV. N. BENSON. 163 



of magnetite toughened the rock against brecciation, which 

 occurred either during the crystallisation of the felspar, forming 

 the kernels around which the last-formed laths might wrap them- 

 selves, or immediately after the consolidation of the felspar, in 

 which case the trachytic structure of the particular fragment 

 would pass unhindered through the magnetitic nucleus. The 

 latter is the more usual feature. The first epoch of crystallisa- 

 tion of the magnetite seems to have been a magmatic one : the 

 magnetite-keratophyre and keratophyre proper must be difiPer- 

 entiates from a common magma, and the peculiar mixed rocks 

 form the transition-zone of incomplete differentiation. After 

 consolidation and brecciation, there still remained a residual 

 magma which consolidated between the fragments. This granular 

 mesostasis may consist of quartz and albite, of quartz, albite, and 

 magnetite, of quartz and magnetite, or of magnetite alone. The 

 last two types of matrix sometimes form in such narrow crevices 

 between the fragments, or in cracks traversing them, that it 

 seems most probable that they are of the nature of hydro-pneu- 

 matolytic veins. In confirmation of this, we may note that they 

 slightly impregnate the rocks on either side of the vein. In one 

 specimen, the mesostasis retained nearly all the magnetite(1188). 

 The nodular segregations of magnetite are quite different from 

 those in the Scandinavian rocks, which, according to Backstrom, 

 are vesicles filled by pneumatolytic deposits of magnetite, but, 

 according to Geijer, are " concretionary bodies in the porphyries, 

 and have crystallised under igneous conditions, and pass into the 

 normal ground mass on the one hand, and into true vesicles on 

 the other."(25,p.715). 



Within the zone of these mixed brecciated keratophyres, lies 

 the main mass of magnetite-keratophyre of the Hyde's Creek 

 complex. It is much more uniform, and brecciation is not so 

 very marked a feature. 'Jhe slaggy, vesicular character is doubt- 

 less due to the former presence of magmatic gases, and the rough, 

 non-amygdaloidal shape of these cavities is, perhaps, explicable 

 on the assumption that the rock moved in jolts by successive 

 brecciation s of almost solid rock, and not entirely by steady 

 viscous flow. 



