55 



The only mineral, to which one might think of uniting 

 ancylite is the weybyeite described by BröggerM. The cry- 

 stals of both these minerals belong to the rhombic system, and 

 both consist chiefly of carbonates of cerium metals. I also 

 started a priori from the supposition that the new Greenland 

 mineral could be identified with the weybyeite. This has, how- 

 ever, proved impossible. The weybyeite crystals resemble small 

 crystals of zircon formed by the fundamental pyramid alone. 

 This may also be said to be the case with the ancylite crystals, 

 but basal sections of the crystals in this position differ from 

 each other. With the weybyeite the extinction-directions are 

 diagonal, with ancylite, on the other hand, parallel, to the 

 borders of the section. The axial ratios calculated for the two 

 minerals are : 



for weybyeite a:b :c == 0,9999 : 1 : 0,64 

 - ancylite a:b:c = 0,916 : 1:0,917. 



Giving the crystals this position, the optic axial plane of 

 ancylite is parallel to the base , that of weybyeite parallel 

 to one of the vertical pinacoids. If the ancylite crystals are 

 given such a position that the plane of the optic axes coin- 

 sides with one of the vertical pinacoids, e.g. the first, {lOO}, 

 and the obtuse bisectrix with the vertical axis , the forms 

 present become 



e = {lio) and d = {lOl}, 

 and we obtain the following axial ratios : 



a:b :c == 0.917 : 1 : 0,916. 

 These axial ratios cannot, any more than the former, be 

 brought into a rational relation to that of the weybyeite. 



If, lastly, the crystal is revolved 90° about the a-axis , d 

 becomes = (llO) and e = (Oll). With the crystal in this 

 position the optic axial plane is parallel to the second pinacoid, 



1) Zeitschr. f. Krystallogr. Vol.16, pag. 650. 



