THE ELECTROMAGNET IN PETROGRAPHY 31 



account of what has been accomplished by the many workers 

 who have used the magnetic method of examination in mineral 

 and rock studies, and to call attention to certain meritorious 

 work, especially by Delesse, which seems to have been unduly 

 neglected by writers of text-books. The account will serve to 

 emphasise the utility of the electromagnet in the examination 

 of grains and fragments of rock-forming minerals. 



2. Historical Review 



During the latter half of the eighteenth century, numerous 

 observations were made by various workers, including Rome 

 de Lisle, Delarbre, and Haiiy, on the magnetic polarity 

 exhibited by crystals of magnetite and haematite. 



Among geologists, H. B. de Saussure early recognised the 

 value of the magnet, and included it in his list of instruments 

 needful to a field geologist. In vol. i. of his Voyages dans les 

 Alpes, published in 1779, he discussed, in a most elaborate and 

 interesting manner, the attractive effect of garnets on a magnetic 

 needle, and described a delicate means for observing this effect ; 

 Saussure even proposed to use this property as a means of 

 estimating the amount of iron in a garnet. He also dealt with 

 the magnetic influence of mountains as observed by a magneto- 

 meter. As an explanation of this influence he put forth the 

 " rock-magnetism " theory, and considered that the cumulative 

 magnetic effect of the ferriferous minerals (hornblende, etc.) 

 of which the mountains were composed, was quite sufficient 

 to account for the effects observed. Further, he indicated the 

 value of the magnetometer as a means of locating bodies of 

 iron ore. (" Cette observation prouve la sensibilite du magneto- 

 metre (1) l'usage que Ton pourroit en faire pour decouvrir 

 des mines d'aimant ou de fer " : loc. cit. tome iv.). 



Incidentally, and as an illustration of the way in which modern 

 authors are apt to overlook the credit due to early workers, it 

 may be pointed out that Haanel 1 attributes the discovery of 

 this method of locating iron ores to Wrede (1843). Again, with 

 respect to Saussure's observations on the magnetic disturbance 

 exercised by mountains, as evidenced in the region of the Alps, 



1 On the Location and Examination of Magnetic Ore Deposits by Magneto- 

 metric Measurements, Department of the Interior, Ottawa, Canada, 1904. 



