nth and Penfidd — Lcmqfordite^ Nsequehanits^ etc. 123 



oal fact's. On the reflecting goniometer the faces in the pris- 

 matic rone frequently yield an unbroken band of reflections 

 of the signal with more prominent parts indicating the position 

 of a unit prism m, 110 and a brachypiuaeoid b 9 010. No re- 

 flections were ever obtained from a macropinacoid and what 

 appears to be that face on many of the crystals is probably a 

 rounding off of the obtuse edge of the prism owing to oscilla- 

 tory combination of the prismatic faces. No sharp reflections 

 were obtained in this zone from either prisms or brachypina- 

 coids, but by measuring from the most prominent reflections 

 approximate values were obtained for the prismatic angle. 

 The faces at the ends of the few crystals at our disposal yielded 

 better results, the dome faces giving fair reflections of the sig- 

 nal so that the angle could be measured with a good deal of 

 accuracy. The basal plane gave in all cases a double reflection 

 of the signal, the two reflections being just about 1° apart, 

 while the reflection from a simple basal plane, just truncating 

 the brachydome, should be midway between them This vi- 

 cinal development of the basal plane can be detected on all of 

 the crystals. The cleavage of the mineral is quite perfect 

 parallel to the prism and less perfect parallel to the base. 

 Parallel to the prism the mineral breaks with a splintery, almost 

 fibrous fracture and no success was obtained in developing large 

 flat prismatic surfaces by cleavage ; with larger crystals and more 

 material one might obtain cleavage prisms which on the goniom- 

 eter would give a very correct measurement of the prismatic 

 angle, but in trying to obtain such prisms, splinters were always 

 loosened which were more or less displaced from their proper 

 position and yielded multiple reflections of the signal so that 

 we could not tell from just what reflection to take the reading 

 of the goniometer. The best measurement of the prismatic 

 angle was however obtained from one of these cleavage prisms, 

 and although not as good as one would desire, it is the best that 

 the material at our disposal afforded. 



The foVms which have been identified on the crystals are : 



c, 001, O; b, 010, i-l; m, 110, /; and d, 011, M. 



The arrangement and development of the faces is shown in 



fig. i. 



For fundamental angles the following were chosen : 



d* d, 011 ^ 011 =49° 6' 



m ~ m, 1 1 /v 110 (both cleavage) = 65 36 



from which the following axial relation is calculated : 



d:b: c=0645: 1: 0'4568 



The following angles were measured independently on six 

 different crystals in addition to the fundamental angles given 

 above : 



