Joly — On the Occurrence of Harmotome at Glendalough. 167 



crystal looks as if terminated with, four smooth planes, and only on 

 very close examination is it apparent that the pyramid is trun- 

 cated and replaced by the four prismatic faces. Many of the 

 crystals are terminated thus at both ends. 



Working with a defective goniometer, the following values were 

 obtained : — 



OaI = 90°; 



I a I ' = 124° 20', mean of nine observations ; 

 I a I = 110° 20', mean of four observations. 

 Dana records harmotome as orthorhombic, and 



iAi = 124°47'; 



IaI = 110° 26'. 



Thus the measurements are evidently sufficiently in accord with a 

 right rhombic prism of 124° 47'. Further, the angle I I agrees 

 satisfactorily with the recorded value. Observations with the 

 polariscope confirms the crystallographic characters ascribed to 

 these faces, but, owing to the generally imperfect translucency of 

 the crystals, are not very definite. 



Specific gravity. — I mentioned opaque, white, lamellar forms. 

 To these optical or crystallographic investigation could not be ex- 

 tended. Thinking they might be a distinct zeolite, it was thought 

 advisable to compare their sp. gr. with that of the other implanted 

 crystals. Otherwise, also, it was evidently advisable to determine 

 the sp. gr. of both forms. 



In a diffusion zone above Thulet's solution, according to Pro- 

 fessor Sollas' method, a fragment of authentic harmotome was 

 placed. On putting in, now, fragments of both the Grlendalough 

 forms, they were found to float exactly in the same horizon with 

 the authentic harmotome. Orthoclase of a sp. gr. 2*51 floated 

 below them, analcite floated much above them, stilbite higher still. 

 By calculation, then, a sp. gr. of 2'46 (Dana 2 , 44-2 , 45) was ascribed 

 to the Grlendalough harmotome. This is a very distinctive test, as 

 the only other members of the zeolite family with so high a sp. gr. 

 are the monoclinic varieties, scolocite and brewsterite. 



