442 



opposed ones were tinged l)lue, while the other ones were oranp:e. 

 Eoei'u sector is opdcuhj huiclal, /oith posltlci'. cluiracter ; the rwinl 

 plane is in evenj sector perpendlcularl// orientated upon the corre- 

 sponding edge (110) : (001). The four quadrants were limited in the 

 centre of tlie basal section by straight borders, corresponding with 

 the edges of the pseudo-tetragonal pyramid ; in every sector the 

 direction parallel to the corresponding edge (110): (001) is that of 

 smaller optical elasticity. 



The plate, wliicli was cut parallel to |100|, showed on very 

 strong enlargement and by the aid of a gypsum-plate with the red 

 of 1^' order, a very fine lamellar structure: the lamellae are super- 

 posed parallel to the faces of the pyramidal, apparently tetragojial 

 limiting forms, while also locally smaller or more extended fields 

 can be discerned, in which the optical orientation appears to be 

 dilTerent and in an orientation, evidently perpendicidar with respect 

 to each other. 



Of these plates the Rontgen- pat terns were obtained in the usual 

 way : the fig. 5a, plate II represents the image, if the plate parallel 

 to |001j is radiated through; it corresponds to the centre of the 

 basal sections, where the four sections are tangent to each other; 

 fig. 6 was obtained by radiation through one single sector, and 

 fig. bb represents the RoNTGENogram, correspondiug to a radiation 

 through the plate, cut parallel to jJOOj. 



Although fig, 5rt appears to be approaching to a much higher 

 degree to real tetragonal symmetry, it is easy to recognize in it the 

 perpendicularly crossed partial figtires of the photographs fig. 1 — 4, 

 but without the formerly mentioned intensive five spots near the 

 centre; and fig. 5/> shows a symmetry with respect to two planes 

 of symmetry, perpendicular to each other, and a binary axis. In 

 fig. (5 it would again be possible to doubt this approach to tetragonal 

 sj'mmetry ; however it seems to be present, and the figure allows, 

 e.g. by direct comparison with fig. 4, to prove that in the radiograms 

 of the /ct'/r?m/-apophyllite doubtlessly several elements of the mono- 

 symmetric American structures are present. From all these peculiarities 

 it seems that we may conclude, that the image of the apophyllite 

 from Iceland approaches only therefore more that of a real tetragonal 

 crystal, because the intergrowth of the monoclinic lamellae is in this 

 case much finer and more regular than in the American species; 

 and with this doubtlessly the other fact is connected, that the 

 Icetaiid-mmerüd looks so much clearer and within larger sectors 

 more homogeneous, than the turbid-looking and opaque American 

 apophyllites. 



