Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/214

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T. G. BONNET ON COLTTMNAK, FISSILE,

regular structure. The same may be observed in the prismatic lava at Torre del Greco (Naples), and at Fingal's Cave (Staffa).

The curved forms of the columns have been so fully discussed by Mr. Scropo and Mr. R. Mallet that I pass them by, merely stating that while some of the cases of slight flexure may be produced by a small motion of the lava-stream, after the formation of the columns, but before it has become perfectly solid, the majority must be due to changes in the form of the surface of what we may call breaking- tension in the cooling mass. It is extremely difficult to explain such complicated cases as the Clam-shell Cave (Staffa), Pouk Hill (Walsall), the Roche Sanadoire (Auvergne), and Jaujac (Ardeche) ; but we must remember that the surface of cooling must be closely related to the surface of the mass, and that partial denudation has often deprived us of the data for that part of the problem. Varia- tion in the conductivity of the superincumbent materials might also produce some effect. Further complication has been sometimes introduced by large fissures, which appear to have opened out in the the mass somewhat anterior to its breaking into columns, and have allowed of a more rapid escape of heat from their surfaces. Thus columns will often be found to curve to a large, more or less hori- zontal joint (see fig. 1).

Fig t l, — Columns of Trachyte curving to a joint (Pic de Saucy, near Cascade of tlie Bore).

A, B. Great joint. C. Minute cross-joints.

The structures more or less parallel to the surfaces of the mass, and consequently generally at right angles to the columnar, have next to be noticed. These may be roughly classified as (a) Fissile and Platy, (6) Tabular, (c) Curvitabular, (d) Cup-and-ball struc- ture.

(a) Fissile. — A distinct tendency to a cleavage, at first sight closely resembling that of ordinary slates, is often visible in igneous rocks. A pitchstone on the north side of the hill of Bunfion (Arran) splits with tolerable ease in one direction. ■ The great intrusive sheet of pitchstone on the Corriegills shore of the same island also has a distinctly fissile character, especially near the upper and lower surfaces, to which the structure is parallel. The same structure may be often observed near the surface of basalt dykes, as on this same shore ; usually, however, it extends only for a few inches into