PRISMATIC STRUCTURE IN IGNEOUS ROCKS 229 



22 mm. in width, with an ill-defined wavy border. This also may 

 have been due to weathering. Within the central eighth of the 

 area appeared 5 amygdules from 2 to 4 mm. in diameter, and filled 

 with a greenish opalescent mineral. Five others, from 1 to 2 mm. 

 in diameter, appeared in the remaining seven-eighths of the area, 

 none being closer than 25 mm. to the border. The section therefore 

 offers no decisive evidence of a differentiation, although markedly 

 different in character from the Bonn column. 



6. Types of cross-jointing in the columns. — A differentiation due 

 to convection might be expected to affect the cross-joints of the 

 columns. The peculiar convex-concave or cup-and-ball joints are 

 seldom found in irregular narrow columns of the typical contraction 

 type, and might have some direct connection with a convection 

 structure. Another type of cross-jointing of columns is the "platy " 

 variety, which is sometimes very regular; its origin has not been 

 satisfactorily explained from the physicist's standpoint. 



Certain special peculiarities of the cross- jointing may also have 

 to do with the mode of origin of the columns. For instance, James 

 Thomson 1 observed that the symmetrical concave-convex joints of 

 columns from the Giant's Causeway have their origin in a small 

 spot or knob which lies at or near the axis of the column, and differs 

 in texture and hardness from the rest of the rock; from this origin 

 the crack has spread outward, as shown by the radial fracture lines. 

 This same form of fracture has just been described above, as occur- 

 ring in the National Museum's specimen from the Giant's Cause- 

 way. Dauzere mentions the same peculiarity in the columns at 

 Murols, and compares it with the core {noyau) which forms in the 

 convection prisms of his wax-salol mixture. 



There seems to be good foundation for the opinion that some 

 sort of original structure is responsible for the spheroidal weathering 

 of columns, and that it is not due solely to the rounding off of 

 jointed blocks by weathering, as some have claimed. Thus Bonney 

 cites numerous examples of spheroids formed from columns which 

 showed no cross-joints whatever. 2 Whether these latent spheroids 

 have any connection with the manner of growth of the column it is 



1 Belfast Nat. Field Club, Ann. Rep., VII (1869), 28-34. 



2 Quar. Jour. Geol. Soc, XXXII (1876), 140-54- 



