fibrous green or brown substance, with a strong double 

 refraction antl moderate refractive index. The pleochroism 

 varies with the depth of colour, the tints being from yel- 

 lowish-brown to pure green, the absorption being in the 

 darker flakes almost total parallel to the cleavage. This 

 Mr. J. A. Thomson considers to be biotite, and it is this 

 biotite apparently which is introduced by solutions into 

 the cleavage planes of the adjacent felspars. This alter- 

 ation to mica is quite different to that developed by inter- 

 action of pyroxene and felspar as described by Parsons. 1 

 In addition to the biotite there may be developed confused 

 brown aggregates of chlorite, or pale green pleochroic 

 spherulites or irregular flakes of the same mineral, the 

 double refraction of which is only slightly higher than that 

 of the plagioclase. With this is often associated grains of 

 carbonates, calcite, magnesite, siderite or combinations of 

 these. Possibly also a little actinolite is formed but its 

 presence could not be definitely proved. 



The rhombic pyroxenes have two main methods of alter- 

 ation. In the majority of cases they pass into actinolite* 

 the alteration taking place at the boundaries and along 

 transverse cracks ; the fibres lie parallel to the cleavages 

 of the pyroxene (see fig. 5). This, however, is best developed 



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