366 C. A. SUSSMILCH AND W. G. STONE. 



patches of a darker and much softer serpentinous material 

 are also abundant. 



Microscopic Description.— Holocrystalline and por- 

 phyritic with a microcrystalline groundmass tending to a 

 pan-idiomorphic fabric. The porphyritic constituent is 

 augite, while the groundmass contains felspar, augite, 

 biotite and apatite. 



Augite. — The augite phenocrysts constitute 30 to 40% of 

 the rock by volume; they have an average size of 1 to 

 2 mm., but crystals up to 4 mm. occur. It is a pale green 

 to almost colourless variety, and occurs in beautifully fresh 

 looking, idiomorphic, well cleaved crystals, sometimes 

 showing typical examples of twinning; and are similar in 

 every way to those occurring in the andesites at the Gave 

 House. A serpentinous material is abundant, occurring 

 in patches of about the same size as the augite phenocrysts. 

 It is pale green to colourless and probably pseudomorphous 

 after the augite. Except that a similar serpentinous 

 material is noticed along cracks and cleavage directions in 

 some of the augites, there appears to be no transition stage, 

 as one would expect to find, between the remarkably fresh 

 looking augite on the one hand, and the complete serpentine 

 patches on the other. 



The groundmass is comparatively fresh with a distinctly 

 holocrystalline texture, the average grainsize being from 

 0*1 to 0*2 mm. It consists of felspar, with lesser amounts 

 o£ augite and biotite, while fine needles of apatite are 

 plentifully scattered through it. 



Felspar is strictly confined to the groundmass, pheno- 

 crysts being typically absent. It has a general lath-shaped 

 appearance and includes both plagioclase and orthoclase, 

 the former predominating. The felspar would constitute 

 about 40% of the rock by volume. 



