134 On the production of the Prismatic Structure of Basalt. 



portion of a prism no hotter than the exterior), then no reason 

 could be assigned why transverse fractures or cross joints should 

 be produced at all however long the prism ; for every prism 

 being free and independent from its summit at S S (fig. 4) down 

 to B B (the lowest point at which splitting is actually taking 

 place*), the contraction of every prism lengthways would be 

 met simply by the shortening or subsidence of the prism itself. 

 But the exterior of every prism is colder than the interior por- 

 tion, the difference in temperature between the exterior and in- 

 terior portions being greatest at some point between B B and 

 S S (fig. 4). Differential strains in approximately the longitu- 

 dinal direction are therefore produced in each prism ; so that 

 taking for origin the first cooled film S S (fig. 4), the outer por- 

 tions contracting most, tend to pull off or upwards or to approach 

 S S in the direction of the arrows as seen in fig. 5, and to leave 

 the central portions C behind for the time. 

 The effect of this inequality of strain is the 

 same in result as that which would be pro- 

 duced by some external force tending to 

 draw upwards or in one direction the ex- 

 ternal parts of the prism, and to leave 

 behind or relatively to push downwards 

 or in the opposite direction the central 

 parts thereof; and if the differential 

 strains be sufficiently powerful to origi- 

 nate transverse fracture, this must com- 

 mence all round the circumference of the 

 prism at some point of its length, and travel 

 inwards until the whole prism becomes divided by a transverse 

 joint, which will happen when the central portions have cooled 

 down to the same temperature as that at which the external 

 portions began to divide transversely ; and such joints will repeat 

 themselves at definite intervals along each prism, proceeding 

 downwards from the first or highest one, Im, fig. 4. We have 

 supposed our illustrative mass of basalt isotropic, contraction for 

 equal decrements of temperature being the same in all directions. 

 Under any given conditions as to the material and mode of cool- 

 ing, the first or uppermost transverse joint cannot occur nearer 

 the top surface S S, fig. 4, than the diameter from side to side of 



* In consequence of the elasticity of the material, the plane passing 

 through all the splitting-points at any instant will be found a little deeper 

 (by a distance which will be nearly constant) than, or in advance of the 

 isothermal couche to which the splitting is due (viz. 600° to 900° Fahr.), 

 just as a cleft produced by a wedge driven into an elastic block of wood 

 extends somewhat further than the point of the wedge itself; this remark 

 also applies to the transverse joints of the prism, hereafter referred to. 



