B. TERRESTRIAL PHYSICS AND METEOROLOGY. 7l 



quence of this cooling, the surface of the volume of plastic 

 rock has arrived at that temperature at which its rigidity 

 becomes sufficient to prevent its yielding to the contractile 

 strains in any way except by its splitting np into much 

 smaller surfaces which can draw off from contact with each 

 other, then the principle of least action, or that principle 

 which as a universal law governs every operation of nature 

 by which a given effect is produced with a minimum expend- 

 iture of work, comes into play ; and in consequence thereof 

 the mass of rock is split up, not into triangular nor into 

 square columns, which might otherwise be possible, but into 

 horizontal prisms. The temperature at which various me- 

 tallurgic slags and the temperature at which actual lavas 

 lose their plastic or viscous condition and assume sufficient 

 rigidity to break, varies between 900 and 600 Fahr. The 

 prisms thus formed, being cooler at their extremities and 

 warmer in the middle of the mass, are thus subject to strains 

 as the cooling process proceeds which break them np into 

 shorter lengths. These lengths slightly exceed the diame- 

 ter of the prism and the fracture separating two adjacent 

 lengths must be of a cup-shaped form, as being also the re- 

 sult of the unequal temperature of the mass. In general, the 

 convexity of the cross-joints will point in the reverse direc- 

 tion to that in which the wave of heat has been transmitted 

 from any cooling surface of the mass. The regularity of the 

 shape of the prisms and their joints will be sensibly affected 

 if there be foreign material or empty cavities existing in the 

 mass of the basalt, since these circumstances affect the rate 

 of cooling. Thus in walking over the tops of large pave- 

 ments of basaltic columns, such as those exposed at the 

 Giant's Causeway and elsewhere, one sees places in which 

 the hexagonal form becomes irregular, and gives place 

 though for a very small extent of surface to quadrangu- 

 lar and other irregular forms, sometimes even triangular or 

 wedge-shaped; but passing on a short distance we find these 

 rapidly return to the normal hexagonal type. Curved or 

 bent prisms must also be produced in consequence of those 

 forces that are brought into play when the bounding sur- 

 faces supporting the whole mass of cooling basalt are them- 

 selves curved. Philosojyhical Magazine, August and Sep- 

 tember, 1875. 



