1887.J ^* [Genth._ 



edges, some show flat hexagonal prisms, upon quartz or porphyry, a few 

 also are groups of minute hematite crystals, together with those of hyacinth- 

 red cassiterite. The most remarkable and only specimen which is large 

 enough to distinguish its form with the naked eye came from the pseudo- 

 morphs of mimetite composition of the Minadel Diablo. 



It is a little group of T™"" X 5™™ in size, composed of dark hyacinth red 

 crystals. There may be ten or more little crystals present, but only a 

 few are perfect enough to show the form, but even these have somewhat 

 curved faces and are not smooth enough and are too much interrupted 

 to allow of an accurate measurement. They are slightly barrel-shaped, 

 apparently hexagonal prisms with hexagonal pyramid and basal plane ; 

 the largest is 3™™ high and of about the same diameter. 



The specific gravity of the group was found to be = 6.49G, which is evi- 

 dently too low, probably on account of the presence of a nucleus of quartz 

 or porphyry around which the crystals may have formed. 



The question presented itself, whether the observed forms were really 

 hexagonal, in which case the dioxide of tin would be dimorphous, or, 

 whether they were resulting from the twinning of tetragonal forms. In 

 order to obtain a conclusive answer, I submitted this group, and also 

 crystalline sands from the washings, received from Mr. Schlenim, to Prof. 

 Gerhard vom Ratli, in Bonn. 



I am greatly indebted to him for his aid in this matter, which was ex- 

 ceedingly difficult to determine on account of the great minuteness of 

 these crystals, so that only one of so much experience and perseverance as 

 he has could attempt to solve this important question. 



He found nearly 135° for the angle made by the apparent basal plane 

 and face of the hexagonal pyramid, which closely corresponds to the 

 angle produced by the first and second tetragonal pyramids of cassiterite. 

 For one prismatic edge he found 112^^ (must be 120° in the hexagonal 

 system), and this angle, too, is the ordinary angle of the geniculated edge 

 of cassiterite twins. Furthermore, he was enabled to pick out crystals 

 from Mr. Schlemm's sands, with which approximate measurements of the 

 angles between the basal plane and the six planes of the apparent hexagonal 

 pyramid could be made, which gave : 134° 10', 135° 15', 135°, 134°, 135°, 

 and 134p, in agreement with the above measurements, as closely as could 

 be expected. 



He writes in his last note, under date of October 26th, 1886, referring to 

 the small group of crystals : " You may consider the determination as 

 quadratic beyond question, assuming an hexagonal habitus by repeated 

 twinning." 



The twin combination resembles figure 163, page 159, in J. D. Dana's 

 "Mineralogy," 1868. I am indebted to Dr. Kcenig for the accompanying 

 drawing, illustrating my group of crystals : 



Besides the localities mentioned, I have received the red cassiterite from 

 Coneto, in the State of Durango, Comanja and Paso de Sotos, in the 



