THE ELECTROMAGNET IN PETROGRAPHY 49 



was somewhat imperfect, and the following result can only be 

 regarded as approximately correct : 



Magnetite (titaniferous) 

 Pyroxene "\ 



Brown biotite 

 Green hornblende | 

 Olivine 



per cent. 



2-9 



• 43"o 



Plagioclases 



Apatite 



Zircon 



Pyrite ) 



~ . > traces 



Quartz j 



per cent. 



54'o 



The non-magnetic group, which, as already stated, consisted 

 almost entirely of plagioclases, was then treated with heavy 

 liquids. In this way the plagioclases and apatite were each 

 isolated. From the zircon and pyrite thus remaining, the zircon 

 was isolated by heating till the pyrite was partly oxidised, after 

 which it was separated magnetically. The zircon thus isolated 

 consisted of crystals of the ordinary type, colourless, and having 

 the characteristic inclusions. In amount they were almost too 

 small to be weighable, forming less than o'oi per cent, of 

 the rock, and yet they proved to be substantial objects for 

 microscopical study. 



Some idea of the effectiveness of this method of dealing with 

 small quantities of zircon may be gathered from the following 

 considerations. The largest zircon crystal obtained had dimen- 

 sions about o*5 x o'i x ot mm. Ten such crystals weigh 

 less than 0*25 mg., though they can be handled with ease for 

 microscopical purposes. Such an amount of zircon obtained 

 from 20 grams of the rock would amount to about o*ooi 

 per cent. 



The above process for determining the presence of small 

 quantities of zircon is to be recommended, as it is a simple 

 one and takes little time. Moreover, zircon in such small 

 quantities appears to be frequently present even in the most 

 basic rocks, such as olivine gabbros and basalts, and it 

 is then practically impossible even to detect its presence by 

 chemical methods. Further, it would appear that zircon in such 

 small quantities generally eludes observation in thin sections, 

 judging from the rarity of the cases in which it is mentioned in 

 descriptions of basic igneous rocks. It should be recalled that 

 Chrustschoff, partly by elutriation, and partly by treatment with 

 acids, isolated small quantities of colourless zircon from many 

 varieties of rocks. One of the rocks he examined was a basalt, 



4 



