358 



University of California Publications. 



[Geology 



ordinary natrolite. Instead of the usual prisms with square 

 pyramidal terminations there are formed small roof-shaped 

 ridges as shown in plate 34, commonly with curved or more 

 strictly broken roof lines and coxcomb-like groups. These are 

 made up of elements that are bounded by a basal plane and unit 

 prism. The smaller angle of the prism generally projects out into 

 the open cavity. In other words, the tendency is to place the b 

 axis at right angles to the wall. The apex of the roof-shaped 

 form is then a prism edge. The radiate groups are produced by 

 laying the elements together along the diverging basal planes 

 whose intersections are roughly parallel. 



More complicated aggregates are common. In certain druses 

 lined with greenish amphibole needles the natrolite occurs as 

 small equant groups looking in the distance as if individual 

 crystals perched on the ends of the amphibole fibres, as shown in 

 figure 2, plate 35. A close examination shows them to be com- 

 plex aggregates in which the tabular elements described above can 

 frequently be observed.. This mode of occurrence is undoubtedly 

 closely related to the botryoidal groups which are occasionally 

 found and which are illustrated in plate 36. Under the micro- 

 scope the structure of the natrolite gangue is rather complex. As 

 viewed on a fracture surface or in section, the radiate groups 

 described show divergent strips that are elongated at right 

 angles to the vertical axis. As the axis is perpendicular to the 

 basal plane the elongation in thin section will always be optically 

 negative. This is just the reverse of that usually seen in diverg- 

 ent natrolite groups and a positive elongation is generally looked 

 upon as a character of diagnostic importance. 14 



In sections parallel to the base is often seen the twinning 

 described by Lacroix, 13 with (110) as twinning plane forming, as 

 it were, a sort of irregular mosaic when viewed with the aid of a 

 gypsum plate. 



A group of natrolite crystals, rare for this occurrence, was 

 found showing the normal development of habit and planes. The 



!* Cf . Rosenbusch, Mikoskopische Physiographic, etc., Bd. I, 2 (1905), 

 p. 178. 



Lacroix, Mineralogie de France, Vol. II, p. 267. 

 isLoc. cit., p. 265. 



