269 



The plates are also traversed by occasional small veins 

 of quartz, which have evidently replaced some of the biotite, 

 leaving the rutile inclusions unattacked and in situ. 



Associated, but not in direct contact with the lode, is a 

 very coarse lighter-coloured mica, which, in thin sections, 

 has a light -greenish colour and is only slightly pleochroic, 

 whilst other sections are practically colourless. Its optical 

 axial angle is about 35°, the optical character negative and 

 /><Cv. It contains a great many more inclusions than the 

 mica of the main lode. It appears to approach the composi- 

 tion of a phlogopite. 



Preparation of Sample for Analysis. 



A quantity of the purer undecomposed mica of the lode 

 was separated by hand-picking. As separated, it contained a 

 considerable quantity of ilmenite and fine quartz as impurity. 

 The mica was then ground to suitable size for treatment with 

 heavy solutions. The next step was to subject it to the action 

 of a strong electro-magnet, which separated a considerable 

 quantity of the magnetic constituents. The residue was then 

 placed in a Brogger separating funnel and a solution of 

 cadmium -boro-tungstate poured in, the contents shaken 

 up, and allowed to stand for some time. The material 

 separated into two layers ; the upper lighter-coloured layer 

 contained a little mica, with a high percentage of quartz; 

 the lower layer contained considerable mica and rutile, with 

 a little ilmenite. The lower layer was detached and again 

 treated with the cadmium -boro-tunp-state, which resulted in a 

 further separation of quartz. The heavy solution was then 

 slightly diluted and agitated, resulting in a rough separation 

 of the mica from, some heavier mica, containing inclusions of 

 rutile and ilmenite. This process was repeated several times 

 until a mica of mean specific gravity was obtained, as free as 

 possible from the presence of these heavier minerals. The mica 

 was well washed with hot distilled water and dried at a 105° C. 



During the above separation, indicators were from time 

 to time thrown into the solution to ascertain the specific 

 gravity. Microscopic investigation revealed the fact that the 

 mica contained numerous rutile needles, and some fragments 

 of mica contained more than others, consequently it was 

 difficult to ascertain exactly the specific gravity of the pure 

 mica. The specific gravity was found to lie between 2"95 

 and 3"07. 



Next, the mica thus separated was treated with dilute 

 hydrochloric acid, washed and dried, to dissolve off any 

 carnotite which may have been present. Thus prepared, the 

 mica showed no apparent radio-activity when tested in a very 

 sensitive electroscope . 



