production of the Prismatic Structure of Basalt. 203 



ceding joint above it ; the vertical components of fracture for 

 every joint below the first one are therefore equal, and the hori- 



fic. 7 



zontal components alone vary, as already described, from / to the 

 axis. The effect of this is simply that the amount of curvature 

 of every joint below the first is rather greater than that of the 

 first, or, in other words, the depth of the cup-shaped articulation 

 of the first joint produced is a little less than that of all subse- 

 quent joints proceeding lower down. 



We have thus proved that contraction alone in the axial and 

 transverse directions is sufficient to account for the production 

 of transverse joints, and their cup-shaped form of fracture taking 

 place at successive intervals as the cooling proceeds along or 

 downwards through the prism, and that, if the material be iso- 

 tropic and homogeneous,, the distance between any two succes- 

 sive joints must be approximately equal for like conditions of 

 cooling; and we can see that the amount of curvature of each 

 joint, and the nature of any curve in any plane passing through 

 the axis of the prism, depend upon the relation that subsists 

 between the length of the piece between two successive joints 

 and the diameter of the prism, and the law according to which 

 heat is lost in directions transverse to the axis of the prism. It 

 would be difficult, if not impossible, to assign precisely the posi- 

 tions and directions of the isothermal couches, as shown in fig. 4 

 and subsequent figures, in so complicated a case of cooling as 

 that which is here before us; but it will be readily seen that 

 neither the form nor the amount of concavity in the joints will 



