THE ELECTROMAGNET IN PETROGRAPHY 41 



4. Lievrite, hedenbergite, limonite. 



5. Augite (15 to 20 per cent, of iron oxides), pleonaste, arfvedsonite. 



6. Hornblende, augite (poor in iron), epidote, pyrope. 



7. Tourmaline, bronzite, idocrase. 



8. Staurolite, actinolite. 



9. Olivine, pyrite, vivianite. 



10. Biotite, chlorite, rutile. 



11. Haiiyne, diopside, muscovite. 



12. Nepheline, leucite, dolomite. 



It has been already mentioned that, owing to variations in 

 composition exhibited by many minerals (especially in point 

 of iron percentage), such lists ought not to be regarded as 

 being accurate. 



Perhaps the most noteworthy feature of Doelter's work with 

 the electromagnet, was that of his experiments on crushed 

 rocks. He attempted the separation of the mineral constituents 

 as far as possible by the electromagnet alone, and stated the 

 results quantitatively. But the fact that he operated on the 

 dry crushings detracts seriously from the merit of the results 

 he obtained. The character and extent of his separations may 

 be seen from the following instances compiled from his results. 



Nepheline Syenite, Antao Island 



Magnetite 5 per cent. 



Augite and hornblende 30 „ 



Nepheline and felspar 65 „ 



Phonolite of Praya 



Magnetite 4 per cent. 



Augite 1 1 „ 



Felspar and nepheline 85 „ 



Compact Basalt, Santiago Island 



Magnetite 9 per cent. 



Augite 39—46 „ 



Olivine 6 — 10 „ 



Plagioclase 32 — 44 „ 



Of the above results, we may take that for the nepheline 

 syenite as possibly accurate. In the cases of the phonolite 

 and basalt, however, we may reasonably inquire what has 

 become of the undifferentiated ground of the rock. Indeed, 

 it is fairly obvious that accurate results cannot be got by 

 physical methods when dealing with hemicrystalline or very 



