656 



DR. J. A. THOMSON OX THE 



[Dec. 1913, 



Pale uralite, preserving the octagonal cross-sections of pyroxene, has 

 been observed in one rock ; while actinolitic outgrowths on the 

 horn blend e-phenocrysts, and uralitic cores within them, in several 

 other rocks, suggest that a small amount of pyroxene was originally 

 present. Biotite is very abundant in those specimens which 



Pig. 3. — Albitized porphyrite, Power-Bouse Reserve, 

 showing cm area of secondary albite, calcite, and epidote. X 45. 



Sencitised felspars Ground: 



Epidote 



Secondary albite 



[Hornblende and biotite, lying in the groundmass, have been 

 omitted for convenience.] 



approximate to plutonic rocks in structure, but assumes diminish- 

 ing importance with the greater abundance of groundmass. It 

 occurs in anhedral platy crystals, generally associated with the 

 hornblende-phenocrysts, or often partly or wholly enclosed within 

 them. The felspars are always euhedral as phenocrysts, and yield 

 quadrate or hexagonal outlines according to the direction of section. 

 Albite-twinning is predominant, both simple and polysynthetic ; 



