﻿Vol. 66.~\ IN THE LEAD AND ZINC VEINS OF GREAT BRITAIN. 315 



stage in a series of complex processes, involving much rearrange- 

 ment of the ores. 



The relations of the opaque minerals are most satisfactorily- 

 determined by metallographic methods, as recently applied to ores 

 by Dr. W. D. Campbell 1 and Mr. C. W. Knight. In the present 

 investigation the specimens were polished on a series of emery discs, 

 and finally on washed rouge or tripoli. The examinations under 

 low powers were made with a petrographical microscope fitted with 

 a vertical illuminator over the objective, light being supplied by a 

 220-volt Nernst lamp. For higher powers a Zeiss metallurgical 

 microscope was used, kindly placed at my disposal by Prof. W. A. 

 Carlyle, in the Metallurgical Department of the Imperial College 



Fig. 6. — Etching figures on 'cubical 9 galena, oriented at right angles 

 to the direction of polishing. (Reflected light : x 200.) 



of Science & Technology. The lenses used were apochromats 

 ot 12, 4, and 2-millimetre focal lengths, and the linear magni- 

 fication ranged from 120 to 700. Of the sulphides examined, 

 the soft galena polishes with a very smooth surface, and gives 

 characteristic etching-pits under the pressure of rubbing. The 

 etching-figures are triangular in outline (fig. 6). Their orientation 

 is unaffected by the direction of rubbing, depending wholly on the 



1 ' The Microscopic Examination of Opaque Minerals ' Economic Geology 

 toI. i (1906) p. 751. J 



