[graham] DAWSONiTE 177 



some way similar to that of nesquehonite. It seems impossible, how- 

 ever, to trace any connection between their formulae which would sug- 

 gest that the internal structures of the two molecules is at all compar- 

 able ; but the intimate relations observed between the properties of the 

 two minerals remain none the less interesting. 



Summary. 



Dawsonite, NajO . AI2O3 . 200^ . 2H2O. 



Orthorhombic, a : b : c = 0.6475 : 1 : 0.5339. 



White prismatic or needle-shaped crystals, striated vertically; 

 also in tufts of radiated fibres. 



Cleavage H (110), perfect, at 65° 51' . H = 3 . G = 2.44. 

 Lustre vitreous, fibrous varieties silky. Colourless or white. Streak 

 uncoloured. Transparent. 



Birefringence, — Y?. very strong; y — a = 0.130. 



Ax. pi. II C (001). Ac. Bis. 1 A (100). Dispersion, p<v, not 

 marked. 



a = 1.466, /3 = 1.542, y = 1.596, for yellow light. 



SVy= 76*' 46'. 2Vr= 76° 33'. 2Ej, = 146^ 27'. 



B.B. infusible. Loses neither water nor carbon dioxide at MCC 

 but both expelled at red heat. Easily soluble in acids with efferves- 

 cence; residue after ignition also dissolves readily in acids. 



Sec. IV., 1908. 12. 



