TERRESTRIAL MAGNETISM. 159 



firmed by Sabine and the French physicists in Lapland m 

 1838. 21 



While in this careful development of the present condition 

 of our positive knowledge of the phenomena of terrestrial 

 magnetism, I have necessarily limited myself to a mere ob- 

 jective representation of that which did not even admit of 

 being elucidated by merely theoretical views, based only 

 upon induction and analogy ; I have likewise purposely ab- 

 stained in the present work from entering into any of those 

 ^eognostic hypotheses, in which the direction of extensive 

 mountain chains and of stratified mountain masses is con- 

 sidered in relation to its dependence upon the direction of 

 magnetic lines, more especially the isoclinal and isodynamic 

 systems. I am far from denying the influence of all cosmical 

 primary forces dynamic and chemical forces as well as of 

 magnetic and electrical currents on the formation of crystal- 

 line rocks and the filling up of veins ; 22 but owing to the 

 progressive movement of all magnetic lines and their conse- 

 quent change of form, their present position can teach us 

 nothing in reference to the direction in primeval ages of 

 mountain chains, which have been upheaved at very different 

 epochs, or to the consolidation of the earth's crust, from 

 which heat was being radiated during the process of its 

 hardening. 



Of a different order, not referring generally to terrestrial 

 magnetism, but merely to very partial local relations, are 

 those geognostic phenomena, which have been designated by 

 the name of the magnetism 23 of mountain masses. These 

 phenomena engaged much of my attention before my Ame- 

 rican expedition, at a time when I was occupied in examin- 

 ing the magnetic serpentine rock of the Haidberg moun- 

 tain in Franconia in 1796, and then gave occasion in 



21 Sabine, On Days of Unusual Magn. Disturbances, pt. i, p. xviii. 

 " M. Bravais concludes from the observations made in Lapland that the 

 horizontal intensity diminishes when the phenomenon of the Aurora 

 borealis is at its maximum" (Martins, p. 461 ). 



** Delesse, Stir 1'association des mineraux dans les roches qui ont 

 un pouvoir magnetique eleve, in the Comptes rendus de I'Acad. dcs Sc. 

 t. xxxi, 1850, p. 806; and Annales des Mines, 4eme Sfirie, t. xv (1349), 

 p. 130. 



-"' Reich, Ueber Gebirys-und Gcsteins-Magnetismus, in Poggend. A itn. 

 Bd. Ixvii, s. iJj. 



