Ransoms.] LawSOHite. 305 



of fine, generally short and scattered, lines intersecting each other 

 at the prism angle. This cleavage is usually much better seen in 

 such portions of crystals as exhibit a blue pleochroism presently to 

 be described, the lines of color in such cases running parallel with, 

 and accentuating, the cleavage cracks. Brachypinacoidal sections 

 also show this cleavage as short, fine lines, much scattered, and 

 intersecting the basal cleavage at 90 . 



The possibility of the mineral being monoclinic was considered, 

 but the coincidence of the axes of elasticity with the crystallo- 

 graphic axes, the angle of 99° between the faces of the basal pinacoid 

 and the prism, and the symmetrical character of the interference 

 figure, would seem to put its orthorhombic character beyond ques- 

 tion. 



OPTICAL PROPERTIES. 



The axial plane is the brachypinacoid, the acute bisectrix 

 emerging on the basal plane. It is positive in character. The 

 orientation of the axes of elasticity is accordingly a = <$, f) = b, 

 C = c. 





















—rr 





















_ .( 



i 



a 



•~ 



t- 



-■" 





\ - 













































'SO 

















i 

 -t- 



Al 







- r 



to 





Figure i. Optical scheme of lawsonite. 



The acute and obtuse axial angles were measured for so- 

 dium light in cassia oil, affording the values 2 H a = 88° 27' and 

 2 H = 103 16', from which the value of 2 V a = 84 6' was cal- 

 culated by the usual formula: — 



sin H a 



