108 



оГ this mineral have been found. These natroh'te crusts gene- 

 rally occur upon the analcime druses. They are consequently 

 younger than these. The natrolite crystals are not over two mm. 

 in length and about V2™™ thick. In general they occur in par- 

 allel position implanted perpendicularly on their substratum. 

 This consists in most cases of a crust of massive natrolite a 

 couple of millimetres in thickness, which sometimes stands on 

 the edge so that crystals can grow out on both sides. The 

 crystals generally stand so thick that they are nearly in contact 

 with each other. As to the combination of faces these natrolite 

 crystals present nothing remarkable. They are bounded by the 

 forms usually occurring on this mineral, viz. 



a = {100}, Ъ = {OIO}, m = {no}, = {ill}. 



As shown by Fig. 3, Plate VI the prism of the third order, 

 m, predominates in the vertical zone. Of the two pinacoids, b 

 generally has the broadest faces. The faces belonging to the 

 pyramid are generally somewhat concave. All the faces are 

 dull or but faintly shimmering; it has not, therefore, been pos- 

 sible to make any accurate measurements on them. In colour 

 the crystals are milk-white and opaque. They are rather brittle, 

 so that they easily break when touched. On the fracture one 

 perceives that only the outer covering is opaque and that they 

 contain a clear and brilhant core. 



Fig, 4, Plate VI shows a section of a natrolite crystal as 

 described above and orientated parallel to the second pinacoid, b. 

 Only the central portion, a, consists of fresh, undecomposed 

 natrolite with tolerably strong double refraction. The line that 

 separates the marginal zone and the central part is quite sharp 

 and straight. The outer zone consists of an aggregate of fine 

 fibres whose longitudinal direction is at right angles to the line 

 between the outer and the central portions. The direction of 

 the least velocity of light is in these fibres longitudinal , as is 

 the case with the fresh natrolite. But the double refraction is 

 considerably weaker in them than in the fresh mineral. 



