142 



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 



