1887.] 'jl- [Genth 



Analysis VII— As,©^ : SnO, : ZqO 

 " of San Antonio ore, 



IX— AsjOj : SnO, : ZnO 

 VIII— " 

 X— " 

 IV_ - 



VI— " 



This shows no rational proportions in the constituents of the cassiterite, 

 and there is no other conclusion than that both arsenic pentoxide, ferric 

 oxide and zinc oxide are admixtures of the tin dioxide, which is easily ex- 

 plained, if we bear in mind the tendency of this latter substance which at 

 the time of its formation, precipitated and retained these oxides. 



c. Cassiterite, pseudomorpJious after Jiematite. — It has already been stated 

 that little red crystals of cassiterite are sometimes found associated with 

 those of hematite. From the sands of Mr. Schlemm a few highly inter- 

 esting, but unfortunately very small specimens were obtained. 



One, about 1""" in size, consists of perhaps a dozen modified crystals of 

 hematite, with the basal plane predominating, in which latter twin groups 

 of pseudohexagonal red cassiterite are implanted ; the occurrence reminds 

 one of rutil which frequently occurs in a similar manner upon the hema- 

 tite (Eisenrose) from St. Gothard. 



Another specimen consists of a group of tabular crystals, radiating from 

 a centre, the whole group 3""" in diameter, which is almost completely 

 altered into reddish-brown brilliant cassiterite, leaving a small core of 

 about 0.5™" in size of unaltered hematite. About ten other pieces of the 

 same kind, although far less perfect, have been observed. 



d. Cassiterite, pseudomorpJious after magnetite ? — A little group of 

 crystals from the ores of the Mina del Diablo — 5 X 3°"° in size, 

 consists of crystals which are apparently isometric octahedrons, together 

 with some botiyoidal aggregations. The crystals are almost black, 

 but mostly show an uneven surface, a color between brownish-yellow 

 and yellowish-green and a waxy lustre, owing to a subsequent coating 

 of reniform, botryoidal cassiterite upon the crystals. A fracture of the 

 crystals shows the dark brownish-red color and lustre of the red cassiterite. 

 The original mineral may have been magnetite. Only three of such 

 minute groups have been found. 



Of great interest in connection with these pseudomorphous forms of cas- 

 siterite are the observations of Mr. Wm. Semmons (published in London 

 in the December number, 1883, of the Natural History Notes), who de- 

 scribes the coating of bismuthinite (BijSj) at the Fowey Consols Mine of 

 Cornwall with thin layers of brownish cassiterite. In a letter dated Lon- 

 don, August 19th, 1886, Mr. Semmons gives me fuller details about this 

 occurrence as follows : 



