1205 
mineral to that of a rhombic crystal, its monoclinic symmetry, how- 
ever, is shown very clearly in the resp. RÖNTGeEN-patterns. Also in 
this case the symmetry of the ROnTGEN-images is quite in agreement 
with the conclusions drawn from the theoretical views. 
§ 4. HL. Monoclinic-prismatic Class. Three plates parallel to (100), 
(010), and (001) were cut out of a beautiful erystal of gypsum from 
Bun Abis, S.W. Africa; their thickness was resp. 0,71, 0,89 and 
0,75 mm. The parameters of this compound are: a: 5 :e = 0,6895 : 
1: 0,4132, and 3 = 81°2’. The obtained RÖNrGrNogrammes are repro- 
duced in fig. 9 to 11 on Plate III, and in stereographical projection 
in fig. 12, 13 and 14 on Plate VI. 
In the same way three plates parallel to (100), (010) and (OOI), 
whose thickness was respectively 1.02, 1,00 and 0,93 mm., were 
cut from a beautiful crystal of epidvie: Ca,HAI,Si,O,,.. The para- 
meters of this mineral originating from Untersulzbach, ave: a:b:e= 
1,5807 :1:1,8057, and p= 64°36". The resulting photos are repro- 
duced in fig. 12 on Plate III, and in fig. 15 and 14 on Plate IV; 
while in fig. 15, 16 and 17 on Plate VI their stereographical 
‘projections are given. 
In this case as well asin both the foregoing, the patterns of erystal- 
plates parallel to (100) and (O01) only show a single plane of sym- 
metry; those corresponding with sections parallel to (O10), a single 
binary axis perpendicular to the photographic plate. This is in full 
agreement with the theoretical expectation; also in these cases the 
observed symmetry of the RÖNTGEN-patterns thus appears to agree 
completely with the conclusions derived from the supposition. 
Finally we reproduce here a pattern of heulandite from Teigars- 
horn on Iceland obtained with a cleavage-lamella parallel to (010), 
parallel to which form this mineral, — whose composition is: 
CaAl,H,(SiO,),+3H,O, and whose parameters are: a: 6:¢c = 0,4085: 
1:0,4788; 8 = 63°40’, is very perfectly cleavable. In the photo 
(fig. 8 on Plate Il) the presence of a binary axis, perpendicular to 
the photographie plate, is clearly discernible. 
§ 5. In this connection a case of dimorphy may find its place 
at the same time, namely that between amphibole and augite (pyroxene). 
As is well known, according to the older conception of TsCHERMAK 
and others, the rhombic, monoclinic, and triclinic pyroxenes, and 
the corresponding rhombic, monoclinie and triclinic amphiboles, 
would be in the relation of polymerides, notwithstanding their equal 
empirical chemical composition: to the amphiboles according to these 
