18 THE ROYAL SOCIETY OF CANADA 



position is in a way comparable with that of arseniosicU'ritc, the ratio 

 of the total arsenate molecules to the ferric hydroxide being in each case 

 1:1. In the case of arseniosiderite, the ratio calcium arsenate : ferric 

 arsenate as usually given in the text-books is 2 : 1. Pul)lished analyses 

 however fail to support this, but show that the percentage of lime pre- 

 sent is never high enough to conform to this theoretical ratio, the cal- 

 cium and iron present in the form of arsenate, as actually found, being 

 more nearly in the proportion 0%: Fe"' = 1:1, or even approaching 

 1:2. It would thus seem more in keeping with the facts to refer 

 arseniosiderite to the general formula (Ca^jFe 2^^8208- Fe^' (OH) ,. The 

 Yukon mineral may then be expressed by the same general formula, 

 with the addition of five molecules of water, and for such hydrous 

 arsenates of iron and calcium the name Yukonite is proposed, in allusion 

 to the locality in which the mineral now described is found. 



It may be remarked that some pitticite (a hydrated ferric arsenate 

 and sulphate of doubtful composition) closely resembles this mineral 

 in colour, lustre and other physical characters. 



The soft yellow ochreous material in which the mineral is embedded 

 was not analysed quantitatively, but it contains the same constituents 

 and has possibly been largely derived from its alteration; except for 

 the fact that it is not fibrous, it might well be arseniosiderite. There 

 is in addition a small amount of an apple or bluish-green mineral which 

 occurs as minute rosettes of radiated fibres within the yellow substance, 

 and also often forms a thin coating or film between the latter and the 

 mineral described above. This could not be obtained in sufficient 

 quantity for an analysis, but a qualitative test indicates that it is an 

 arsenate. As seen under the microscope, the needles are strongly 

 pleochroic, greenish-yellow for light vibrating along their length and 

 bluish-green for transverse "vibrations; between crossed niçois they 

 usually show inclined extinction at angles up to 40°, and compensation 

 takes place when the quartz wedge is inserted across theii" length. In 

 all these characters, as well as in its general appearance and occurrence 

 as fine radiated fibres, this mineral resembles syniplesite (Fe'^gAsjOg. 

 SHjO) and there can be no doubt that it is to be referred to this species. 

 So far as the writer is aware, the occurrence of symplesite in North 

 America has not been hitherto recorded. 



Mineralogical Laboratory, 

 McGill University, 

 Montreal. 



