146 



T. G. BONNET ON COLUMNAR, FISSILE, 



gular prisms, which are traversed by so many cross joints as to 

 make the rock almost platy in structure ; then, after a distance of 

 about a yard, where the normal joints become rather more frequent 

 and somewhat confused, a series of magnificent columns commences, 

 which run with great regularity, and are singularly free from cross 

 joints, the shafts often being unbroken for a distance of several 

 yards. On approaching a contact with the tuff on the lower side of 

 the mass a platy structure again sets in. 



This tabular structure, it will be observed, is roughly parallel to 

 the bounding surface of the mass, as is the bedding in the case 

 noticed under Burntisland Castle. Intrusive dykes also sometimes 

 show a tabular or roughly platy structure running parallel to their 

 bounding surfaces. 



(c) Curvitabular Structure. — I proceed next to describe a struc- 

 ture which I have not often noticed myself, and do not remember to 

 have seen described by others — which, for the sake of a name, I may 

 denominate curvitabular jointing. As an example of it I will de- 

 scribe the basalt of the Plateau de Prudelle, above the valley of 

 Yillar (Auvergne), a sketch of which is engraved by Mr. Scrope, in 

 plate vi. of his work on the Auvergne. This basalt crowns a rib or 

 promontory of granite, resting on this or on a thin intervening bed of 

 sandy (and ashy ?) marl containing rolled felspar crystals and a few 

 bits of scoriaceous basalt. Above the marl comes a rather irregular, 

 hard, " ropy "-looking, brown band, probably only the baked crust 

 of the marl, which is united irregularly to the base of the basalt 

 flow. This is very rough, slaggy and scoriaceous, in places re- 

 sembling an agglomerate of cinders, sometimes partly fused together, 

 sometimes loose. This crust is about a foot thick, and passes rather 

 suddenly into a somewhat slaggy-looking basalt, with a few 

 elongated vesicles and occasional irregular vertical joints. This 

 continues for about 2 feet ; and then the curvitabular structure sets 

 in rather suddenly. The mass is traversed by a series of more or less 



Fig. 6. — Diagram of Curvitabular structure (Plateau de la Prudelle). 



