410 JTeates and Ayres — Occurrence of Plattnerite 



by them. The specific gravity of artificial lead dioxide is 

 variously stated to be 8*902 (Herapath),* 8-933 (Karsten),f 

 8-756, S-897 (Playfair and Joule),+ and 9-045 (Wermcke).§ 



The sizes of the nodules vary from &"xl"Xi" to 8"x64/ X 

 4t/\ one of the largest masses weighing 15 lbs. 1 oz. Im- 

 planted on many of them are crystals of the white pyromor- 

 phite ; and, as has been mentioned above, these nodules, when 

 broken open, are speckled with this mineral. Some of these 

 crystals are brown, being coated by a film of plattnerite ; and, 

 until this film was cut through and the true nature of the 

 crystals determined, they were thought to be plattnerite crys- 

 tals. As the plattnerite was originally described! as being 

 rhombohedral, occurring in hexagonal crystals, and as Greg 

 and Lettsom^[ had suggested, that it might possibly be pseudo- 

 morphous after pyromorphite, thin sections were made, and, at 

 the writer's request, these were kindly, studied under the 

 microscope by Mr. Geo. P. Merrill,** who reported, that he 

 could find no evidence that any alteration of the pyromorphite 

 into plattnerite had taken place. One large specimen of platt- 

 nerite, when broken open, showed sharp angular fragments of 

 milk-white quartz, which formed a breccia with the plattnerite 

 as the cement. 



In breaking open specimens to obtain the fresh fracture, the 

 writer observed, that, here and there, would be one, which was 

 distinctly fibrous, in part, the fibers running from the dense, 

 black, cryptocrystalline material to irregular, small cavities, 

 which were, more or less, filled with a bright yellow-ocher. 

 These fibers seemed to terminate in minute jet-black crystals, 

 penetrating slightly into the ocher. These were so imperfect, 

 however, that the form could not be made out, even with a 

 strong lens. The fibers, when examined, were .found to be 

 plattnerite. A few drusy surfaces, which had been the sides 

 of small fissures, were observed ; and these, on examination 

 under the microscope, the writer found to be made up of 

 slender crystals, to all appearances tetragonal, and strikingly 

 like certain cassiterite crystals from the Dolcoath mine in 

 Cornwall, except that they were opaque. Examination proved, 

 that these crystals were plattnerite. At the writer's request, 

 Mr. Edward F. Ayres of the TJ. S. Patent Office very kindly 

 undertook the delicate crystallographic work required. His 

 description is hereto appended. 



* Phil. Mag., Loudon, etc., lxiv, p. 321. 



f Schweigg. Jour., lxv, p. 394. 



\ Mem. Ch. Soc, iii, p. 84. 



§ Jour. Ch. Soc. (II), ix, p. 306. 



I Loc. cit. *[ Loc. cit. 



** Curator. Dept. of Geology, TJ. S. Xatioiial Museum. 



