BY W. G. WOOLNOUGH. 527 



Fluidal structure is not universally exhibited by the microlites 

 of the base, even in cases where it is very marked macroscopically 

 in the arrangement of the phenocrysts. 



Felspars of the first generation present idioniorphic sections up 

 to 4 mm. X 3 mm. in area. They have suffered considerably from 

 mechanical forces, and from corrosion by the magma, but are 

 otherwise beautifully fresh. The zoning in ordinary light, which 

 has been described in other rocks, is even more marked in this 

 one. There seems to be a great tendency for the zonally-arranged 

 inclusions to split up into concentric zones with clear spaces 

 between. The zoning is also marked by differences of refractive 

 index, the variation being normal — that is, the central portions 

 having higher refractive indices than the peripheral zones. The 

 refractive index of all 'parts is greater than that of Canada 

 balsam. Both the ordinary cleavages and the prismatic parting 

 are strongly developed. 



In polarised light it is found that albite twinning is strongly 

 developed, combined in many cases with that after the Carlsbad 

 law. Pericline lamellae are by no means uncommon. Sections 

 in the zone perpendicular to -j 010 J- give symmetrical extinctions 

 up to a maximum of 35^ for the central portions and 25° for the 

 peripheral zones, with a maximum difference of 15° between the 

 two portions of a Carlsbad twin. Highly satisfactory measure- 

 ments in a section parallel to -| 010 [- give- 13"^ for the outer 

 zones, and - 22*^ for the kernel. These results areveryconcordant, 

 and show that the felspar may be called labradorite, the different 

 zones varying between the two limits of that species. The outer 

 zones are near Abj^ An^, the central portions near Ab.^ An^. The 

 unindividualised inclusions in the felspars are exactly like those 

 described in the rocks from iS^amulowai and Voma, especially the 

 latter, and are certainly due to similar causes. The individualised 

 inclusions are referable to augite in grains and prisms mostly from 

 0*001 to 0-003 mm. in greatest length, but in some instances as 

 much as 0-075 mm., and grains of magnetite. Some of the larger 

 augite lumps contain small but perfect octahedra of magnetite. 



