140 T. G. BONNET ON COLUMNAR, FISSILE,
18. On Columnar, Fissile, and Spheroidal Structure. By the Bev. T. G. Bonnet, M.A., F.G.S., Fellow and Tutor of St. John's College, Cambridge. (Bead February 23, 1876.)
The subject of the columnar jointing and spheroidal structure of rocks has of late received considerable attention, the most im- portant papers on the subject being those by Professor James Thomson* and by Mr. K. Mallet f. The former explains the colum- nar structure by a contraction of the mass in cooling, and advances a novel theory to account for the cup-and-ball structure observed in cross-jointing, viz. — that the fracture commenced at the centre, owing to a longitudinal tensile stress, starting often from a " small [included] mass of stone different in texture and in hardness from the rest" of the rock, and proceeded outwards towards the peri- phery. The cause of this tensile stress he considers to be, probably, chemical action set up by infiltration of water, which has produced an expansion of the outside of the column, so that the outer part, growing longer, has strained and finally snapped the interior. The spheroidal structure, often manifested in decaying basalts, he considered not to be " an original concretionary structure, but due to decomposition penetrating from without inwards in blocks or fragments, into which the rock has been fissured."
Mr. E. Mallet commences his paper by an ingenious mathematical demonstration of the cause of hexagonal fracture in the contracting body. He then passes on to consider the cup-and-ball structure. This he regards as a further product of contraction from loss of heat in a prism which is now cooling from the sides as well as from an end, so that the curved surface of the joint is always concave to the end which is losing heat ; and he regards the spheroidal structure as the result of the residual forces of contraction which yet remain in the imperfectly cooled prismatic blocks into which the column is divided.
That columnar structure was clue to contraction was clearly pointed out many years since by Mr. Scrope in his admirable work on the AuvergneJ; so that the confused statements in subsequent text-books of geology well deserve the severe comments of the above- named authors. With their explanation I fully concur ; and the .demonstration of Mr. E. Mallet seems to me unanswerable. I shall therefore pass rapidly over this part of the subject, merely calling attention to one or two points of interest in connexion with columnar structure. As, however, I entirely dissent from Professor J. Thomson's conclusions in the rest of his paper, and differ to some extent from those of Mr. Mallet, I venture to think there is yet room for a few remarks on the subject — one which I have for some time past lost no opportunity of studying.
- Eeport of Belfast Naturalists' Field Club, 18G9-70.
t Phil. Mag. ser. 4, vol. 1. pp. 122, 201. X Volcanoes of Central France, p. 92.
