236 



L. V. Pirsson — Monleiiiie. 



the faces and to striations due to a repetition of the crystal 

 form in parallel position, a considerable series had to be examined 

 before any sharp reflections could be ob- 

 tained. No sharp reflections could be ob- 

 tained from any clino-pinacoid, since from 

 the separation of cleavage plates it was in- 

 variably too rough to reflect well. Finally 

 one crystal was found which gave very fair 

 and distinct reflections in the prismatic 

 zone and from the orthodomes. Another 

 gave good reflections in the zone of symme- 

 try. From these the following (supple- 

 mentary) angles were taken as fundamental : 



c^t = 001 ,s 201 = 63° 40' 

 t*s = 201^201 = 50 12 

 s~l = 201^450 = 36 07 



and from these we calculate the axial ratios 



a : b:c:: -40101 : .1 : -42623, angle /3=88° 30-J' ; 



and for heulandite we have, 



a: b: c:\ -40347 : 1 : -42929, angle /?=88° 34£'; 



adopting the orientation given the latter species by Des 

 Cloizeaux. Only one other satisfactory measurement could be 

 made : 



Calculated. Measured. 



ZaZ = 450 a 4oO = 53° 15' 52° 44' 



52° 33'. 



The only difference then between the mordenite and the 

 Jones' Falls crystals of heulandite is that the prism occurring 

 on the latter if taken as I (110) requires the similar prism on 



the mordenite to be (450) i-\ . 



The crystals occur attached at one end upon their prismatic 

 faces. They form groups from growth parallel to the clino- 

 pinacoid and are also somewhat radially disposed. The cleav- 

 age is eminently clino-diagonal and the luster of the clino- 

 pinacoid is pearly, so that cleavage fragments resemble small 

 nacreous fish scales. Under the polarizing microscope, using 

 cleavage plates of the mineral which furnish sections parallel 

 to the clino-pinacoid, it was found that the plane of the oj)tic 

 axes is normal to this face. The direction of extinction is 

 negative, according to the scheme adopted for the plagioclase 

 feldspars and inclined about 15° to the clino-diagonal axis. 

 This axis of elasticity = c and h = a. The optic angle is large 

 and it is uncertain whether c or a is the acute bisectrix ; the 

 double refraction is weak, high polarization colors being shown 



