these. In some cases the olivine is replaced by a c< >n fused 

 aggregate of green pleochroic fibres with slightly oblique 

 extinction, which are probably actinolite. Crystals so 

 altered resemble the "pilite" pseudomorphs after olivine 

 in lamprophyres. In other cases the mineral is of faint 

 pleochroism, but the extinction is straight. The bire- 

 fringence is strong and a is parallel to the length of the 

 fibres. This mineral seems most probably anthophyllite. 

 The central portion of the grain may be filled in with finely 

 matted and less highly coloured anthophyllite or it may be 

 pale green pleochroic platy substance like the biotite of the 

 diallage, though not so highly coloured. Again the central 

 portion of the grain may be of talc in colourless or very 

 pale green flakes or fibres of high birefringence, occurring 

 in rectilinearly bounded sectors, which seem to have no 

 definite orientation. The fibres of neighbouring sectors 

 interlace with mutual compensation. Occasionally there 

 is a distinct sign of multiple twinning in the talc plates. In 

 dunites this alteration takes place along bands of easy 

 passage for solution. The lateral portions of these show 

 pale to medium green actinolite and anthophyllite, the 

 central portions very pale talc. The intergranular partings 

 are marked by dusty carbonates. The bands of black oxides 

 deposited in irregular cavities are partly or completely 

 resorbed during this type of alteration. 



Another type of alteration has several varieties. Quartz 

 is present in varying amount and manner. There are 

 actiiHilite-quartz pseudomorphs after olivine in which the 

 quartz forms small irregular, often interlocked grains, 

 either scattered through the pilitic material or forming a 

 central area rimmed round and crossed by bands of pilite, 

 (fibrous amphibole). Sometimes the quartz forms a plate 

 occupying the whole area between the actinolite bands; in 

 other instances it may be in interlacing groups of parallel 



