THE ELECTROMAGNET IN PETROGRAPHY 49 
was somewhat imperfect, and the following result can only be 
regarded as approximately correct : 
per cent. per cent. 
Magnetite (titaniferous) 2°9 Plagioclases 
Pyroxene Apatite 
Brown biotite : Zircon : 21540 
Green hornblende | ° ‘eae Pyzite: —}) 
Olivine Quartz }§ paces 
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‘or 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 05 x o11 x o'r 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‘oo1 
per eent. 
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 
