46 



meridian direction, like the current of sap in a living tree, we 

 need not be surprised that the wall of the fractures, cannot al- 

 ways be refitted ; their ruptured sides are altered by the chemical 

 action in a very short time ; the southern parts of which are often 

 seen penetrating into the northern by anew cleavage formed sub- 

 sequently to the filling of the cracks, as represented in Plate XI. 



Miners are well aware that the sides of veins often bulge out 

 in defiance of all mechanical resistance : it requires a consi- 

 derable practical knowledge to keep them open to extract the 

 mineral, particularly in very wet ground. When the splits 

 happen to be in a north-west direction, the masses of rocks on 

 the western side are generally forced northivard 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 M that are only apparent. (Plate 

 XII.) In Cornwall the majority of the splits are north-west, as 

 described in Plate VIII. : consequently 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 De- 

 von, has been shifted northward on the western side nearly half 

 a mile. In the vicinity of Tavistock and Callington 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 direc- 

 tion and inclination of the mineral veins or transverse fractures ; 

 but this is a mistake : the movements of the masses are quite in- 

 dependent of the cracks, and would be the same had they not ex- 

 isted. Nor does it follow that the dislocated veins should be always 

 (i heaved " on the side of the obtuse angle, as generally supposed, 

 because this depends on the angle of the fracture itself. (Plate X.) 



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

 manifest when we examine the nature of the mechanical disturb- 

 ance. Admitting the magnetic force to act in the meridian, the 

 direction of the oblique splits destroys the parallelism or uni- 

 formity of the polar forces ; consequently the masses presenting 





