214 Mr. R. Mallet on the Origin and Mechanism of 



regions, though no solution of the facts has hitherto, so far 

 as the writer is aware, been given. In fig. 17 such con- 

 vergent prisms are shown separating one of those huge mush- 

 room-shaped protrusions of basalt which have welled up through 

 fissures (as in fig. 13) and have also overflowed the surface, 

 examples of which are numerous in Saxony and elsewhere. In 

 such a case cooling is almost wholly confined to the top sur- 

 face, and the convergent prisms with the interruptions already 

 explained run to the bottom or near it, resting there on 

 irregular fragments. Towards the edges of such masses (figs. 

 17, 18, 19) the basalt, by rapid cooling, may be not prismatic, 

 but amorphous ; or if prismatic, the prisms may be more or less 

 confusedly curved, on principles to be hereafter referred to. 

 If (as in fig. 18} a hollow or valley be filled, whether by welling 

 up from a fissure or supplied from some lateral flow which has 

 introduced the mass of basalt beneath a thick covering of detrital 

 and badly conducting material, then cooling may take place 

 proceeding chiefly from the bottom, and so slowly that the con- 

 traction of the upper part of the mass may be sufficiently re- 

 lieved by the subsidence of the upper portions en masse, in which 

 case the columnar structure will be almost wholly developed by 

 cooling from the under surface, and at a certain height above 

 this columnar structure will become evanescent ; and all the re- 

 mainder of the mass, except for a few inches or feet in depth, in 

 contact with the detrital covering, where it will be broken up 

 into irregular fragments, will be found to consist of massive 

 basalt alone. Examples of this passage from prismatic into 

 massive basalt are to be found in all basaltic regions. 



Inasmuch as the splitting into straight prisms always takes 

 place perpendicular to the cooling surface, it is obvious (on 

 inspecting fig. 20) that in the event of basalt filling a valley of 

 generally wedge-shape section, its top surface being covered by 

 tufa or other badly conducting substance, so that the cooling of 

 the mass takes place almost wholly by conduction to the sides of 

 the valley, the prisms will be found in a sloping direction and 

 generally perpendicular to both flanks of the valley respectively, 

 being more or less irregular or confused as the valley -flanks 

 themselves are so. If (as in fig. 21) the valley be broader and its 



f/c . so 



FIC. si 



flanks more or less convex, and the mass of basalt be cooled 



