Tie ELEC TROMAGNET 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 Romé 
de Lisle, Delarbre, and Hatiy, on the magnetic polarity 
exhibited by crystals of magnetite and hematite. 
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 sensibilité du magnéto- 
metre (1) l’usage que l’on pourroit en faire pour decouvrir 
des mines d’aimant ou de fer”: Joc. 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? 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. 
