Pirsson and Wells — Occurrence of Lead 'hillite. 223 



This agrees pretty closely with the observations of Descloi- 

 zeaux* and of the other investigators mentioned in this article. 

 Bertrandf gives 2 Ey=72° for the mineral from Matlock, and 

 as it exhibited other optical anomalies mentioned by him the 

 occurrence appears to need confirmation by chemical analysis. 



As usual the optic angle diminishes rapidly with increase of 

 temperature. Plates placed on the heating attachment for the 

 microscopic of Fuess became uniaxial when the thermometer 

 registered about 100°C. ; the plates themselves, being exposed 

 to the air, were at a somewhat lower temperature. During the 

 process the turning lamellae disappear as noted by Hintze^: and 

 Mugge§ and the plate becomes uniform in polarization. In 

 the course of a day or so the uniaxial character disappears and 

 the normal one is resumed. This change seems to be due to 

 molecular rearrangement as the mineral does not suffer any 

 chemical change, as shown by the careful experiments of 

 Hintze.| 



Etching figures. — The monosymmetric character of leadhil- 

 lite is well shown by the form and disposition of the etching 

 figures produced on the basal surface of cleavage plates by 

 very dilute nitric acid. 



After the action of the acid has continued a few moments 

 and the surface has been rinsed and cleaned with water, it is 

 found to be covered with delicate markings. With high 

 powers of the microscope these are resolved into shallow pits 

 whose outline on the surface is the projection of a nearly 

 equilateral spherical triangle. 



One side of the triangle is tangential at its middle point to 

 the trace of the orthopinacoid (100) on the base (001) while 

 the opposite angle is bisected by the symmetry plane. The 

 sides of these depressions curve towards each other as they 

 descend and either end in a dihedral angle or in a small plane 

 which truncates it. 



Thus the character of the figures produced by etching is 

 precisely similar to that of the micas described and figured by 

 Baumhauer^f and shows the monoclinic symmetry in the same 

 way. 



When the action of the acid is more rapid, the etchings run 

 into one another and in this way the twinned character of 

 cleavage plates, the arrangement of the twinned bands and 

 different individuals is quite clearly brought out. 



Chemical composition. — Although leadhillite has been fre- 

 quently analyzed, its chemical composition has remained some- 



* Propr. Opt., 2, 38 N. R. 72, 1867. + Loc. cit. 



\ Pogg Ann , clii, p. 259, 1874. § Jahr. f. Mia., i, pp. 6B, 204, 1884. 



I Pogg. Ann., clii, p. 256. 



\\ Zeit. fur Kryst., vol. iii, p. 113, fig. 12, 1879. 



