176 



The pyroxene of the least altered rock is a pale green diopside, containing 

 only the incipient stages of diallagic structure. It is almost colourless in thin 

 section, and precisely resembles the pyroxene of some of the Scotch tertiary 

 gabbros. The normal pyroxene of the Lizard gabbros is a well-characterised 

 diallage, often possessing a metallic lustre on the plane of easy separation. 

 The replacement of diallage by hornblende is a common feature in the Lizard 

 rocks. The secondary hornblende may be compact, uralitic or actinolitic. 

 Actinolitic hornblende frequently forms radiating fringes round the uralitic 

 variety, or round a crystal of diallage partially changed to uralite. Where 

 the rock-mass has been subjected to deformation the diallage has frequently 

 been replaced by confused aggregates of grains and fibres of hornblende. 

 The secondary hornblende may be green, brown or colourless. Green 

 varieties are very common. There seems no reason to believe that any of 

 the hornblende in the Lizard gabbros is an original product. 



The olivine may be either fresh or serpentinized. The fresh mineral is 

 colourless, and traversed by cracks along which magnetite has been deposited. 



In many rocks the olivine, where it occurs in contact with felspar, 

 is surrounded by one or more zones. Sometimes unaltered olivine is separated 

 from unaltered plagioclase by a narrow zone of a compact mineral, having 

 the pleochroism of a rhombic pyroxene. This mineral is perfectly fresh, and 

 the junctions with the olivine on the one side and felspar on the other are 

 quite sharp. Considerable portions of the zone show uniform optic orientation. 

 At other times the zones consist of fibrous minerals which show a rough 

 tendency to a radial arrangement with respect to the olivine. This appears 

 to be more common round altered olivines, and has much more the aspect of a 

 secondary formation. In the most common case two zones may be recognised. 

 The inner one consists of a colourless fibrous mineral which gives straight 

 extinction. The outer one in contact with the felspar is green, and possesses 

 all the microscopic characters of actinolite. Magnetite-dust usually occurs 

 between the colourless needles of the inner zone, but is altogether absent from 

 the outer zone. Seeing that these zones agree in character with those of the 

 Rosswein rock, described by BECKE, it seems probable that the mineral of the 

 inner zone is anthophyllite. 



Fig, 34 



