production of the Prismatic Structure of Basalt. 215 



simultaneously from its top and bottom and from the convex 

 flanks of the valley or hollow, the prisms will tend, on the whole, 

 to perpendicularity towards the centre of the valley, being formed 

 in one or several ranges separated by angular fragments; while 

 at the convex flanks the prisms may assume curvilinear forms, 

 on principles now about to be more particularly explained. 



So far the writer has treated only of the formation of straight 

 prisms; but curved or apparently bent prisms must also be 

 produced by the play of those forces which have been already 

 explained when the bounding surfaces supporting the mass of 

 cooling basalt are curved, or the mass of such form as is suitable 

 to their production. 



Whatever be the form of a cooling mass of basalt whose tem- 

 perature is supposed originally uniform throughout, the wave of 

 heat which is first lost is parallel to the external contour of the 

 mass ; and if the mass be of such a form that successive iso- 

 therms are parallel to each other and to the original contour, as 

 in the tabular masses that we have been considering, the prisms 

 into which it splits, whatever be their direction in space must be 

 straight and normal to the external contour or bounding surface 

 of the mass. But if the mass be of such a form that successive 

 isotherms taken in descending order (as ii, kk, It, fig. 22) do 

 not remain parallel, the prisms (as pp', p ! p", &c.) can be no 



longer straight, because as successive isotherms at the splitting- 

 temperature diverge from each other in one direction and con- 

 verge towards the other, and as the splitting takes place normal 

 to the isotherm or couche which is at the splitting-temperature, 

 so the axis of each prism continually changes in direction as 

 the splitting passes through successive divergent isotherms, as 

 may be seen in figs. 22, 22 A, 22 B, — the first representing a 

 mass with the top surface level, and the bottom a curve convex 



