522 Geology and Magnetism. 



a north-west direction, the masses of rocks on the western side 

 are generally forced northward more than those on the eastern 

 side : if the splits be towards the north- east, the contrary effect 

 takes place; that is, in real heaves, because a great number are 

 called " heaves" that are only apparent. In Cornwall the majority 

 of the splits are north-west, as described in Plate VIII. :* conse- 

 quently all the principal " heaves" of the country are to the right, 

 the western masses having shifted northward more than the eastern. 

 The red sandstone and carbonaceous series, intersected by a split 

 near Tiverton in Devon, has been shifted northward on the western 

 side nearly half a mile. In the vicinity of Tavistock and Calling- 

 ton similar northward movements are observed. There is another 

 great northward " heave" near Redruth, produced by the great cross 

 course traversing the North Downs. The direction of the " heaves" 

 is generally expressed by right and left, because the same expression 

 serves on approaching them on either side. Some suppose that the 

 nature of " heaves" depends on the direction and inclination of the 

 mineral veins or transverse fractures ; but this is a mistake : the 

 movements of the masses are quite independent of the cracks, and 

 would be the same had they not existed. Nor does it follow that 

 the dislocated veins should be always " heaved" on the side of the 

 obtuse angle, as generally supposed, because this depends on the 

 angle of the fracture itself. 



The cause of the above order in the dislocated masses is made 

 manifest when we examine the nature of the mechanical disturbance. 

 Admitting the magnetic force to act in the meridian, the direction 

 of the oblique splits destroys the parallelism or uniformity of the 

 polar forces; consequently the masses presenting the largest trans- 

 verse bases to the south will be propelled northward at a greater 

 rate than the others. (Plate XII.*) 



Having lately made a very extensive investigation of the princi- 

 pal mines of Cornwall, Wales and Ireland, with a view of instituting 

 a comparison between them and those of America, this part of our 

 subject was carefully attended to, and the result has fully confirmed 

 our previous opinion that the greater number of the veins on the 

 large scale have been " heaved" and filled simultaneously. The 



• See Note, p. 518. 





