1887.] 



35 



[Genth. 



Mineral Cabinet in Vienna, and at my request, to give me his opinion. Dr. 

 Aristides Brezina wrote me under date, Vienna, July 31, 1886 : 



" The pseudomorphs are evidently after mendipite* and permit to de- 

 termine the heretofore incomplete elements of this mineral with a fair ap- 

 proximation : 



m : m' =: 77° 19', mean of four measurements with the 



hand goniometer — mendipit measured 77° 24'. 

 d : d' ^ 102° 23', mean of four measurements with the 



hand goniometer — 

 from which it follows that the elements are rhombic, 

 a :b:c =0.8002:1 : 0.9948 (i =0.8044) ifm = (110); 

 d = (101). 



" At the same time the pseudotetragonal character is 



peculiar ; the difference of the parameters b and c lies 



within the errors of observation, but the habitus of the 



crystals speaks for the rhombic system." 



I thought it best to give these different views, trusting 



Fig. 10. that the future may furnish, by the discovery of the 



unaltered mineral, together with the pseudomorphs, the true explanation 



of this doubtful subject. 



The analysis of these pseudomorphs shows them to be mimetite in 

 which a small quantity of the lead is substituted by calcium. Even the 

 purest have an admixture of clay, containing a minute quantity of 

 dioxide of tin. 



Analyses 1 and 2 were made by myself with material selected from the 

 best crystals and apparently quite pure, 3 and 4 were made by Mr. Harry 

 F. Keller, with apparently pure crystals. 



* I have just received a letter from Dr. A. Brezina, dated : Wien, Aphl 7, 1887, in 

 which he admits it as an oversight to have considered the mimetite pseudomorphs as 

 being after mevdipite, and states that there is now nothing to prevent the acceptance of 

 my original views that the primitive mineral was anglesite. F. A. GENTH. 



University of Pennsylvania, April 20, 1887. 



