[gkaham] DAWSONITE 167 



Meanwhile, Dr. Harrington-"' had found other specimens of daw- 

 sonite at the Montreal reservoir, in a dyke which is probably a continua- 

 tion of that in which the mineral had originally been discovered near 

 McGill College. In the course of further analyses of this new material 

 he found that the percentage of lime varied very considerably, some 

 specimens containing as much as 16.85^ CaO, while at the same time 

 the ratio of the other constituents was always constant. He therefore 

 concluded that the lime was not an essential constituent, but was pre- 

 sent in the samples he took for analysis in the form of admixed calcite; 

 and recalculating his analyses on this assumption, he was able to show 

 that the Canadian dawsonite can be represented by the same formula 

 as that obtained by Friedel for the material found in Tuscany. 



A third occurrence of dawsonite, at Ténés, Algeria, was subse- 

 quently described by J. Curie and G. Flamand^ : and specimens from 

 this locality resemble those from Tuscany so closely in their mode of 

 occurrence and appearance, that they can only with difficulty be dis- 

 tinguished from them. The analyses also agree well with the formula 

 Al,03.Na20.2C0,.2H20. 



So far as I am aware, these three are the only localities at which 

 dawsonite has been found; but the position of the mineral as a well de- 

 fined species has been fully established. 



Crystallography and Optical Properties. 



The capillary nature of the crystals, as found in Tuscany and 

 Algeria, renders the determination of their symmetry by measurement 

 on the goniometer an impossibility; and although the Montreal material 

 is more suitable owing to its comparatively coarse crystalline structure, 

 yet terminated crystals are almost never found, the striated blades (or 

 fine needles lying in parallel position) being intergrown and reticulated 

 in an apparently irregular manner. 



In his first paper on the subject, Harrington stated that " as re- 

 gards the crystalline form, I am uncertain, though it is probably mono- 

 clinic, with the inclination of the principal axis about 75°." 

 Descloiseaux^ also stated that the crystals are probably monoclinic, and 

 was able to make a rough determination of some of the optical pro- 

 perties. He found they had a strong birefringence, a wide optic axial 



• Can. Nat. New Series. X. 2. p. 84. 1881. 

 " Ann. Fac. Sc. de Marseille, II, 2, p. 49. 

 ' Bull. Soc. Min. p. 8. 1878. 



