Feb. 1900. Crystal Forms ok Cai.cite — Farrington. 235 



of these forms together with the rhombohedron /, - | R, as seen on a 

 crystal in the Museum collection. There is also present one plane 

 of the negative rhombohedron /,-2R. 



Some measurements made on this crystal with the contact goni- 

 ometer are as follows : 



Measured. Calculated. 



v. I - 2131 : 0445 = 43°-44' 44° 6' 



v :/= 2131 : 0221 = 35°~38' 37° 41' 



e : / = 01T2 : 0445 = n°-i2° 12 2' 



e xv — OlTa : 2131 = 45°-52° 50° 36' 



As has often been observed on calcites from other localities, the 

 planes of the rhombohedron c,- l /2 R are, in this crystal, striated 

 parallel to the edge r/r'. They are also somewhat rounded. The 

 curious sporadic occurrence of the plane / and the accompanying 

 imperfect growth of planes seems to be quite common for Joplin 

 crystals showing the above combinations. At least the writer has 

 noticed the same appearance on several other similar but smaller 

 crystals. It would be interesting to know the nature of the molecular 

 forces which would uniformly produce degradation of this sort. The 

 planes of the scalenohedron v on the crystal are in general flat and 

 bright, but show peculiar markings suggesting stages of growth. 

 These markings are quite irregular in form and seem to have no 

 angular values whatever. They follow, however, so far as they have 

 any arrangement, the lines of the cleavage rhombohedron. Partings 

 along planes of this rhombohedron are to be seen through the crystal 

 and the surface of attachment is usually characterized by cleavage 

 planes. 



Other crystals of Type 2 are more highly modified than the one 

 just described. Fig. 1, PI. xxix, represents one such crystal now in 

 the Museum collection. Here the primary scalenohedron v, -J- R3 is 

 modified by the scalenohedrons n, -f- R£ and w, -J- | R2, also by 

 the positive rhombohedronsr, -(- R and M, -{-'4R, and by the negative 

 rhombohedron /,-|R. Only about half the complete form is present 

 in the specimen. In Fig. 2, PI. xxix, is shown the completed ideal 

 form which would be produced by these combinations. Some of the 

 measurements made on this crystal with the contact goniometer are 

 as follows : 



Measured. Calculated. 



v : r =2131 : iofi = 30 29 2' 



v : M = 2131 : 4041 = 18 19 24' 



r : M = ioil : 4041 = 31 31 \o x /l ' 



v : l = 2131 : 0445 = 44° 44 6' 



