CHAPTER XXVI 



MAGNETIC ROCKS 



The literature on the subject of the magnetism of rocks is very 

 extensive,! and even if I was capable of doing so, any attempt to 

 deal generally with this complicated phenomenon would be out of 

 place here. Zirkel in his characteristically thorough fashion has 

 reviewed the subject in his general work on petrography, but since 

 the date of the last publication of that book, 1893-94, the literature 

 has been much increased and the subject has from time to time 

 been opened up in scientific periodicals, occasionally in ignorance 

 of the labours of those that have gone before. Here, the local 

 magnetisation of rocks is alone considered, the general question of 

 earth magnetism not being entered into. 



According to Zirkel one of the earliest known observations of 

 this phenomenon was made by Bouguer, the French geographer, 

 whilst he was engaged in the measurement of a degree of the 

 meridian in the vicinity of Quito in 1742. Alexander von Hum- 

 boldt, however, was one of the first to attract general attention to 

 this subject by the announcement of his discovery in 1796 of a 



' A good list of references to the early German authorities on the subject is 

 given in the American Journal of Science and Arts for 1831, vol. 20. . . . Zirkel 

 in his Lehrbuck der Petrographie (1893, vl- i- P- 5^5) gives most of these and 

 many more recent. . . . Harker in his paper below named refers to a review of 

 the earlier literature in Verh. naturh. Vereins. Bonn, 1851, vol. 8, and to a more 



complete bibliography by Meli in Boll. Soc. Geol.Iial. 1881, vol. 9 British 



Association Report in 1889 by Professors Riicker and Thorpe on the Magnetic 

 State of the British Isles. . . . Nature for August and September, 1894, &c. . . . 

 Harker on magnetic disturbances in the Isle of Skye, Proc. Cambr. Philos. Soc. 

 vol. 10, part 5. . . . Skinner in Proc. Cambr. Philos. Soc. May, 1894. . . . Clark in 

 Journ. Roy. Instit. Cornwall, 1890-93. . . . Folgheraiter in Frammenti con- 

 cernanti la geqfisica, Rome : referred to in Nature, July 27, 1899, and Nov. 8, 

 1900. 



