production of the Prismatic Structure of Basalt. 205 



It remains here to explain the play of forces by which the lip 

 or edge of the cup adjacent to the solid angles of the prism is 

 so frequently found broken off, or strained so as readily to break 

 off, in something of the peculiar form shown in fig. 8 in vertical 

 section, and in the plane of the joint in fig. 9. It will be re- 



collected that at the moment of the production of the cup-joint 

 the material at the concave side a is still at a somewhat higher 

 temperature than the mass b from which it has just become de- 

 tached by a fracture, the sides of which are kept in absolute con- 

 tact by the superincumbent weight of b and those above it. 



Cooling still proceeds, and with it contraction until both b 

 and a have attained the same temperature; but the form of the 

 cup-shaped fracture has been impressed upon it at a higher tem- 

 perature as respects the lower block a, which therefore differen- 

 tially contracts as to its cup-shaped cavity upon the lens-shaped 

 surface of b which fills it ; hence strains transverse to the prism 

 and in the plane of the joint are produced, the effect of which is to 

 tend to split off portions of the exterior parts of the cup a; and 

 this action is most effective at and about the solid angles of the 

 hexagon, where the surfaces of the cup- shaped fracture have the 

 most effective frictional grip of each other, i. e. where the angle 

 of the fracture itself is largest with respect to a plane passing 

 through or near the lip of the cup, as c d, fig. 8. This play of 

 forces may either break off altogether three or the whole of the 

 solid angles of the hexagon at the lip of the cup, or may only 

 visit so severe a strain upon the rigid basalt at and about these 

 parts as may cause a plane of weakness by which, at some sub- 

 sequent period, by any slight extraneous force such as frost &c, 

 the fracturing off of these fragments may be produced, as seen 

 in figs. 8 and 9. The curves assumed by these fractures are 

 always such as indicate the play of forces as above described, 



