530 Henry C. Richard.'^ : 



consisting of soft iron about 1| in. wide and ^ in. thick, are 

 slotted so as to move over screw-clamps which fit into the ends 

 of the limbs. By this means the pole-pieces can be adjusted 

 in any desired position. 



The electro-magnet is connected with tTie ordinary lighting 

 circuit of 200 volts alternating current, the latter being con- 

 verted into direct current by means of a Noden -valve arranged 

 by Mr. Grayson, of the Geological Department. Suitable resist- 

 ances were obtained by means of ordinary incandescent lamps, 

 6 of which, of varying capacities, were used, giving an amperage 

 ranging from ^ amp. to 3 amps., the voltmeter indicating 10 

 volts for the latter value. 



PRErARATIOy OF THE SAMPLE. 



The fresh representative sample was first crushed into pieces 

 about the size of peas by means of a small jaw-crusher, this 

 material was then crushed up in a steel mortar, so that it would 

 pass through a sieve with openings about .2 nnn. square (20 

 sieve), but remain on a -10 sieve with apertures about .1 mm. 

 sq. This size was found most convenient for the separation of 

 the larger flakes of mica. After the mica had been removed 

 from this it was further crushed so as to be caught on a 60-sieve 

 with apertures .06 mm. sq. 



The larger size was more convenient for the separation of the 

 mica, while the smaller size was required for the hypersthene. 



Separation of the Minerals. 



It was found that the separation of the mica from the 

 hypersthene by means of a magnet could not be done very 

 satisfactorily, but in transferring the powders in cardboard 

 boxes it was noticed that the flaky biotito had a tendency to 

 stick to tlic bottom of tlic box, while the other rounded grains 

 rolled off easily. This ])Ocu]iarity of the mica was utilised, and 

 proved especially valuable, for while the magnet served to sepa- 

 rate the mica and hypersthene from tlie other minerals in the 

 rock, the gentle shaking in cardboard boxes with one end cut 

 away, served to separate the mica from the hypersthene. 



