Vol. 67.] IN THE OLD EED SANDSTONE OF MONMOUTHSHIRE. 



4G7 



Minute augite-prisms (of the third generation) with 

 an average length of O05 mm. and breadth of O'Ol mm., nearly- 

 colourless, or of a pale greyish-purple or purplish-brown, and 

 showing longitudinal striation and faint cross-parting, are abundant. 

 Minute scales and plates of deep reddish-brown biotite often 

 cling to the octahedra of iron-ore ; and, filling up the spaces between 

 all these crystals, is a colourless or pale yellowish-grey or cloudy 

 substance, usually isotropic, but in places faintly birefringent with 



Fig. 3. 



[A=Analcite ground-mass, enclosing augite-prisms, apatite-needles, iron-ore, 

 and biotite. Towards the centre is a clearer space occupied by allotrio- 

 morphic analcite. X 240 diameters. 



B = Part of ocellus or steam-cavity, with sub-idiomorpbic analcite, carbonate- 

 rhombs, and pale-green chlorite. X 240 diameters. 



C = Decomposed augite-phenocryst, overgrown by augite-prisms. X 100 dia- 

 meters. 



D = Corroded quartz, together with olivine-and augite-crystals, in a monchi- 

 quite ground-mass. X 40 diameters.] 



pale neutral tints, and enclosing very minute needles of apatite. 

 Its low refractive index, absence of colour, and isotropic character 

 suggest the mineral analcite, probably as a primary constituent, 

 and evidence will shortly appear which strengthens this view 

 (fig. 3 A, above). 



