BY D. MAWSON. 463 



viduals averaging 1*5 mm. diam., porphyritically scattered through 

 a groundmass; (b) A younger generation represented by a 

 microcrystalline, somewhat pilotaxitic, granular groundmass. 

 Minerals 2^^'esent (in approximate proportions by area): — 



1st Gen. 2nd Gen. Total. 

 Felspar ...intermed. labrad. (Ab^ An J 



,, ...basic andesine (Abg An2) 



Pyroxene ... 



Olivine ... 



Magnetite... (generations not separable) 



Apatite ...(minute quantity only). 



100 



Felspars of the first generation are represented by labradorite 

 (Abi An^) which exhibits the usual twinning, and is much 

 marked by cleavage cracks. Inclusions are numerous and show 

 a decided inclination to elongation in the direction of the prism 

 zone; brown glass, pyroxene, magnetite grains, serpentinous- 

 looking matter, and, less often, liquid enclosures are to be noted. 

 The crystals are typically developed in aggregates averaging 

 2 mm. diam., in which the individuals are bunched in an obscure 

 radial fashion. This is a variety of the glomero-porphyritic 

 structure described by Prof. Judd;"^ this case is distinguished by 

 the fact that the aggregates are composed of felspars only. A 

 typical felspar glomerulef is shown in the photograph (Pl.xxiii. 

 fig. 4); in it the clear area within the zone darkened by increased 

 abundance of inclusions is an assemblage of first generation 

 labradorite crystals, whose multiplicity is rendered evident under 

 crossed nicols (Pl.xxiii. fig.5). These felspar glomerations were 

 no doubt developed, freely suspended, at a time preceding the 

 final ascent of the lava; this crisis is indicated by the narrow 

 peripheral zone in which, owing to very rapid crystallisation, | 

 inclusions are extremely abundant. Outside this latter dark 

 zone, there appears a narrow marginal band of clear glassy felspar 



* Q.J.G.S. xlii. 71, 1886. 



t Glomerule is a botanical term, but can be equally well applied to 

 such mineral agglomerations. 



ij: Under high power magnification the labradorites show up with a 

 rounded and somewhat ragged outer border, suggesting that possibly corro- 

 sion was the cause of the darkened zone. 



