Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/218

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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).