72 



DR. A. HOLMES OX THE PBE-CAMBRIAN [vol. . lxxiv,. 



The minerals present are quartz, orthoclase, and microcline 

 (including perthitic varieties), oligoclase, bright-green monoclinic 

 pyroxene, dark-brown or green hornblende, straw-coloured biotite,. 

 apatite, sphene, zircon, magnetite, and calcite. The pyroxene is* 

 frequently bordered by a frayed edging of amphibole, which is 

 sometimes brown, pleochroic in tints of brown and yellow, but 

 more generally dark-green, pleochroic in tints of dark green, 

 yellow-green, and blue-green. Along the cleavage-planes fibrous- 

 inclusions of the green variety are also developed, while the brown 

 amphibole occurs in addition as ragged plates that tend to con- 

 gregate around the margins of the pyroxene individuals. Biotite 

 is rare, and is absent from many of the specimens. With the- 

 coloured minerals, the abundant accessories, sphene and apatite,, 

 are chiefly associated, while calcite in irregular blebs is present 

 mainly as inclusions in the felspars. The form of the mineral, its. 

 presence in orthoclase and microcline, and the entire absence of 

 any signs of alteration in the rocks, make it impossible to interpret 

 the calcite as secondary. The association of the rocks with 

 arnphibolites and crystalline limestone leads one to suggest that 

 the calcite represents a recrystallized residue of calcareous material 

 picked up b}' the magma during its passage from below, the 

 greater part of such material having been completely absorbed to 

 form the pyroxene and amphiboles. It is significant that specimen 

 No. 207, which contains less quartz than the other members of the 

 series, is also richest in calcite and amphiboles. 



Table VIII. 

 Mineral Composition of Aughtic Intrusions (by Volume). 



Minerals. 



Artgite- 

 Granite. 



Sawa Cave. 



No. 27. 



Angite- 

 Granite. 



Ampicihi 

 Crossing. 

 No. 208. 



Augite- 

 Adamellite. 

 Amp ii' ih i 

 Crossing. 

 No. 207. 



Atigite-Quartz 

 Diorite. 



Nrassi Basin. 



No. 28. 



Quartz 



Microcline"] 

 Pertliite )> 

 OrthoclaseJ 

 Oligoclase 



22 



49 



18 



9 



o 



18 • 

 56 



8 



14 



1 



1 

 2 



14 



35 



32 



12 

 3 

 2 

 2 



7 

 4 



53 

 31 



1 

 4 



Andesine 



Pyroxene 



Amphibole . 

 Biotite 



Accessories 



Specific gravities. 



2-67 



2-67 



2-70 



2-78 



The augite-granite of the -Sawa caves occurs as a sill-like in- 

 trusion bounded by hornblende-gneiss, and cut by pegmatites rich 

 in magnetite and ilmenite. As the mineral composition (stated 



