346 Canadian Record of Science. 



length of the prism. In some instances the double refraction 

 is sufficiently strong to give -rise to the greenish-yellow, 

 yellow and pink colours usually seen in thin sections of this 

 mineral., but in others, and almost invariably in very thin 

 sections the mineral shows the deep blue interference 

 colours characteristic of Zoisite. It was thought at first 

 that both minerals were present, but a more careful study 

 of the slides showed that the blue colour was given by 

 thinner parts of individuals which elsewhere polarize in 

 yellow tints, the blue colour appearing as border around the 

 little bays or cavities in the crystals to be described further 

 on, and where, therefore, the epidotewas thinner than else- 

 where. Since, however, normal epidote has a sufficiently 

 strong double refraction to give brilliant yellow interference 

 colours even in the thinnest sections ordinarily attainable, 

 it is probable that this is a variety poor in iron, and thus 

 approaching Zoisite in composition, these two minerals 

 being dimorphic, their formula being identical, except 

 that in epidote a portion of the alumina is generally replaced 

 b}' ferric oxide. The absence of the usual pleochroism in 

 the mineral points to the same conclusion. 



Associated with some but by no means with all of these 

 crystals of epidote are little individuals of allanite. These 

 are sometimes very Mriall and of a more or less irregular 

 shape, but frequently have a good crystalline form consisting 

 of a prism elongated in the direction of the b axis and gen- 

 erally having what are probably pyramidal terminations at 

 one extremity. The plane of the optic axis is at right 

 angles to the longer axis of these crystals. It has a high 

 index of refraction, possesses a distinct zonal structure and 

 is pleochroic, the colours being as follows : — 



ft — Light yellowish brown. 



b — Purplish brown. 



C — Pale yellowish brown. 



The light passing through the crystals parallel to a is of 

 nearly the same colour as that passing through parallel to 

 t . The colour is not so intense as is usual in allanite, al- 



