BY D. MA.WSON. 583 



(IB) A more typically developed vein of this class is that 

 enriched by a fair proportion of alkali-iron silicates(Pl.xlix., fig.l ). 

 These are most numerous crossing the dark-coloured syenite of 

 the westernmost quarries. Though the sides are by no means 

 plane, they preserve a well-marked parallelism, definitely 

 indicating contraction fissures in which evidence of lateral move- 

 ment of the sides is absent. The vein filling is uniformly coarse 

 and abruptly differentiated from the fine-grained regirine-syenite 

 traversed. Large crystals of sanidine reticulate from one side 

 to the other, though seldom extend completely across; the inter- 

 spaces are largely occupied by eegirine and acmite-like pyroxene 

 with less frequent arfvedsonite and grains of leucoxenised ilmenite. 

 Uralitisation and carbonation are seldom observable; decomposi- 

 tion of the pyroxene, when evidenced, generally having attacked 

 the regirine-augite centres which are occasionally found as a 

 brownish-green chlorite-like substance. Interspaces are numerous, 

 lined by secondary chalcedony and fibrous quartz, often with a 

 central filling of granular quartz. 



It is supposed that this type of vein had a similar origin to that 

 preceding. Iron molecules were likely derived from a similar 

 source as the ingredients of the sanidine; the late crystallisation 

 of the alkali-iron silicates can no doubt be ascribed to their 

 superior solubility. There is, however, just a possibility that 

 the iron minerals were introduced from a distant source at the 

 close of the sanidine crystallisation period; against this is to be 

 urged the absence of any corrosion or zoning phenomena in the 

 sanidines, indicative of such a change in physical conditions. 



(2) Those of larger dimensions both as regards width and 

 lateral extension. Examples examined and classed under this 

 heading averaged 3 cms. wide. In these amphibole is abundant. 



(«) Veins that have suffered but little from attack by secondary 

 circulating active vapours (PLl.,figs.l and 2). 



These have a fresh appearance, clear idiomorphic sanidines 

 contrasting strongly with the black ferriferous minerals. The 

 best examples were got in a quarry about 200 yards west of 

 Loverid^e's main workings. 



