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in the composition of the original rock and partly to secondary causes. 

 The serpentinisation has been produced by percolating solutions which, 

 in some cases, certainly oxidised the iron and thus gave rise to the red 

 varieties, and in other cases exercised a reducing effect. By means 

 of these solutions the original minerals were not only decomposed in situ, 

 but a considerable amount of migration of certain chemical constituents 

 was rendered possible as is shown by the formation of veins of chrysotile, 

 steatite, calcite and chalcedony. The manner in which these solutions 

 penetrated the rock-mass was doubtless determined by joints produced partly 

 in connection with the original consolidation of the peridotite, and partly in 

 all probability by the earth-movements that have affected the districts since 

 the formation of the original rock. Again, in certain localities, as for example 

 near Nare Head, on the east side of Gerran Bay, Cornwall, the serpentine 

 has been converted into a soft soapy schist by earth-movements acting after 

 the formation of the serpentine. It appears clear to the present writer that 

 most of the districts in which serpentine is known to occur in Britain 

 have been profoundly affected by regional metamorphism ; and if so it 

 becomes a matter of great importance to determine the relation between 

 the times of intrusion, serpentinisation and regional metamorphism. 



Minerals of the Serpentine Rocks. 



Serpentine. The most important is of course serpentine itself. In thin 

 sections this appears either yellow, green or colourless. It is frequently 

 either stained red by ferric oxide or rendered more or less opaque by the 

 black oxides of iron. As already stated the distribution of the iron oxides 

 marks out the original structure of the olivine, when the serpentine has been 

 derived from that mineral. When examined under crossed nicols serpentine 

 splits up, as a rule, into double-refracting bands and isotropic portions. The 

 double-refracting bands intersect each other, and thus produce a network 

 which corresponds with the original structure of the mineral from which 

 the serpentine has been produced. The isotropic portions lie in the centres 

 of the meshes. The existence of double-refracting and isotropic portions 

 of one and the same substance is thus explained by ROSENBUSCH. The 

 minute structure of serpentine is fibrous. The individual fibres have definite 

 optical characters which can be determined in the case of those aggregates 

 in which the corresponding crystallographic planes of the fibres lie parallel 

 to each other ; as for example in chrysotile. They are biaxial with a large 

 optic axial angle and a negative bisectrix at right angles to their length. 

 Where the fibres are arranged with more or less parallelism the serpentine 

 exhibits double refraction ; where, on the other hand, the serpentine is 

 isotropic the fibres are confusedly intermingled and compensation takes 

 place. When the serpentinisation has proceeded from any plane the 

 fibres tend to arrange themselves at right angles to this plane. 



Bastite, the serpentinous modification of enstatite, possesses a distinct 

 lamellar structure, determined by the pinacoidal cleavage of the mineral from 

 which it has been derived. In it the parallel arrangement of the individual 



