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. 



