PLATE II. 



FIG. I. 

 SERPENTINE. 



COVERACK COVE, CORNWALL. 



Magnified 25 diameters. Ordinary light. 



This figure shows, in addition to the serpentine-substance, a large amount of 

 unaltered olivine (1) occupying the central portions of the meshes of serpentine. Adjacent 

 grains of olivine extinguish simultaneously over large portions of the slide, as the stage 

 is rotated between crossed nicols, thus showing that they are parts of one and the same 

 original crystal, and that their present isolation is merely the result of the development 

 of serpentine along original cracks. 



The greenish yellow serpentine-substance may be resolved under the microscope, by 

 the use of a high power, into fibrous or scaly aggregates which, under crossed nicols, 

 sometimes give a faint but definite reaction, extinguishing light parallel with the axes 

 of the fibres, and sometimes appear isotropic. The serpentine-substance is often coloured 

 along the cracks by the presence of hydrated ferric oxide. 



The slide illustrates the relative order in which the minerals of peridotites are 

 affected by the agents producing serpentine. Thus the olivine is seen to have been 

 much more affected than the pyroxene. (2) When the latter mineral is examined under a 

 high power it is seen that the alteration commences along cracks and is indicated by a 

 very fine parallel stria tion which runs outward from the cracks into the clear substance 

 of the crystal to a greater or less extent. 



The opaque black substance (5), seen below the comparatively unaltered pyroxene, is 

 magnetite. 



FIG. II. 

 PICRITE (?). 



MENHENIOT, CORNWALL. 



Magnified 25 diameters. Ordinary light. 



This figure shows serpentinous pseudomorphs after olivine (6), augite (7) and 

 magnetite. The augite when examined under a high power occasionally shows the 

 lamination characteristic of diallage. Adjacent portions extinguish in the same position, 

 showing that they belong to large ophitic masses. 



Under a high power the greenish yellow serpentine-substance is seen to contain 

 minute colourless rnicrolites which appear needle-shaped, fusiform, lozenge-shaped, or 

 lath-shaped in different sections. The long sections give a maximum extinction of 

 about 17. Rosenbusch (Massige Gesteine, p. 530) doubtfully refers these forms to a 

 grammatite-like amphibole. The lozenge-shaped sections sometimes correspond to 

 cross sections of the hornblende prism, and this confirms the suggestion of Rosenbusch. 

 The microlites are sometimes arranged in two directions, more or less at right angles to 

 each other. They are too minute to be represented in the figure. 



Lath-shaped sections of pseudomorphs after felspar are sometimes seen to penetrate 

 the ophitic plates of augite. The affinities of the rock may be indicated by describing it 

 as having been originally an ophitic dolerite (diabase), in which olivine was present in 

 great abundance and felspar only to a very small extent. 



