160 TERRESTRIAL MAGNETISM. 



form, I have no less designedly avoided the attempt to 

 infer geognostical connections between the direction of 

 great mountain-chains or stratified rocks, and of the 

 magnetic lines, more especially the isoclinal and isody- 

 namic lines. I am far from denying the possible 

 influence of cosmical primeval forces, of dynamic and 

 chemical forces as well as of magnetic and electric 

 currents, on the formation of crystalline rocks and the 

 filling up of veins ;( 222 ) but seeing the progressive 

 movement, accompanied by change of form, of all the 

 magnetic lines, it cannot be supposed that the position 

 which they may be found to occupy at any particular 

 time, can throw any light on the relations of direction 

 of mountain-chains upheaved at exceedingly remote 

 but very different epochs, or on the foldings or corruga- 

 tions then taking place in the gradually hardening, 

 heat-exhaling crust of the earth. 



There are indeed other relations, not affecting the 

 earth's general magnetism, but only of a very partial 

 and local character, subsisting between geognostic and 

 magnetic phenomena, which may be called " mountain- 

 or rock-magnetism." ( 223 ) I was much interested by 

 these when examining in 1796 (previous to my depar- 

 ture for America) the magnetic serpentine rock of the 

 Haidberg in Franconia, and they then gave rise in 

 Germany to a good deal of literary debate. They 

 present a series of problems very accessible to observa- 

 tion and experiment, in which there remains much to 

 be determined. The intensity of the rock-magnetism 

 may be tested in detached fragments of hornblende- and 

 chloride-slate, serpentine, syenite, dolerite, basalt, mela- 

 phyre, and trachyte, by experiments of deflection aiid 



