D NOTES ON THE OCCURRENCE OF STILBITE, 



Its hardness, tested at right angles to its cleavage, is above 3, 

 calcite being easily scratched by it. Want of a sufficiently 

 delicate balance prevented me from making an attempt to deter- 

 mine its specific gravity. The crystals are subtranslucent to 

 opaque. 



Before the blow-pipe the mineral gave the following reactions : 

 it exfoliated, swelled up into curiously shaped white ramifications, 

 fusing easily to an opaque white enamel. Moistened with cobalt 

 nitrate and strongly ignited, the assay gave a somewhat dull blue 

 mass, indicating presence of alumina. In the closed tube it yield eel 

 water readily. The powdered mineral was decomposed by hot 

 hydrochloric acid, leaving after evaporation the silica as a some- 

 what slimy powder. The filtered solution, after super-satuiation 

 with ammonia, gave with oxalic acid a distinct white precipitate 

 of oxalate of lime. 



I next examined a thin cleavage section under the microscope. 

 Its appearance by central illumination when magnified 50 diameters 

 is shown in PI. I., fig. 1. Bright orange bands, more or less recti- 

 linear, and of varying degrees of intensity of colour, traverse the 

 section of the mineral parallel to each other. (The greater or less 

 vividness of the tint, no doubt, depends on the thickness of the 

 section at various points, as well as on the mass of pigment 

 injected.) These bands are crossed at right angles by others 

 having either the same colour or a brownish tint. Between these 

 coloured stripes appear colourless or faintly yellow portions, while 

 everywhere, but especially in the deeply coloured regions, groups 

 of black dots are visible, often arranged into lines running parallel 

 to the orange bands mentioned above. Irregularly scattered over 

 the colourless or faintly yellow parts of the slide are small patches 

 of a bright yellow or orange tint, encircling a greater or less 

 number of small black particles. At the point marked A in PI. I., 

 fig. 1, these crystallites are arranged in lines concentric with the 

 contour of the surrounding colour patch. Prof. Zirkel, in his 

 " Beschaffenheit der Mineralien und Gesteine,"* states that these 



* Op. ciL, p. 167. 



