BY B. G. ENGELHARDT. 7 



black spots are the pigment which gives to stilbite its various 

 shades of colour, and that they are microscopic crystals of either 

 gothite, limonite, or perhaps red hematite, the mineral itself being 

 originally colourless. 



PL I., fig. 2, represents an almost colourless section of stilbite, 

 only a few yellow spots being visible, but the orange bands are 

 almost totally absent, while comparatively few of the black 

 crystallites are present. In a similar specimen, unfortunately 

 lost by an accident in mounting, I observed some beautiful 

 dendrites of a bright sulphur-yellow, and as perfectly developed 

 as the macroscopic dendrites of manganese oxide so often found 

 on the cleavage planes of schists, slates, «tc. 



The occurrence of these dendritic aggregates tends to prove, to 

 my mind, conclusively, that the pigment of red stilbite entered 

 the mineral by the process of secondary infiltration of a solution 

 of hydrated per-oxide of iron, derived from the hydration of the 

 magnetite in the surrounding basalt. The solution has spread 

 itself between the thin laniime composing the prisms of stilbite, 

 having found its way through the hair-like cracks (due to shrinkage 

 caused by the drying-up of the fluid in which the zeolite crystal- 

 lized) which can be seen traversing the section in irregularly 

 curved lines. 



Between crossed Kicols the mineral proved to be anisotropic, 

 suffering four extinctions in a complete revolution of the section. 

 It is also very slightly pleochroic, the different tints darkening 

 feebly when the section is rotated above the fixed polariser. 



