154 ON COLUMNAR. EISS1LE, AND SPHEROIDAL STRUCTURE.
generally form at the points where the spheroid's surface was nearest to the outside of the column, because the shell would be rather weaker there.
The independence of the structures is to some extent confirmed by the fact that a thin plane lamina of rock sometimes exists between two spheroids in a column.
All the divisional structures, therefore, which can be observed in igneous rocks are, if the above considerations be admitted, to be re- ferred to one and the same cause, viz. contraction of the mass while cooling. It is not, of course, denied that in some cases effects may have been produced by subsequent chemical or mechanical action (as the true cleavage in certain of the Welsh felstones) ; but it is con- tended that those above described are all the result of one and the same cause.
Discussion.
Prof. Ramsay inquired whether Mr. Bonney had observed the cup- and-ball structure in narrow dykes at right angles to the planes of cooling.
Mr. Koch stated that he had tried many experiments bearing on the subject of prismatic and spheroidal structure. Slag at 1500° C, when suffered to run into water, has a perfectly smooth surface, and exhibits distinct prismatic structure. Fragments of ironstone placed on the sole of a furnace cracked off in shells, leaving spheres ; and quartzite under similar circumstances flaked off in the same way.
Mr. Judd noticed the interesting bearing of the numerous facts described in this paper on the theories concerning the production of columnar structures propounded by Prof. James Thomson and Mr. Robert Mallet. He, however, deprecated the introduction by the author of the question of the fissile structure of phonolites (which microscopic examination proved to be the result of peculiarities in the arrangement of the crystalline constituents of the rock) as having any relation to the structures resulting from contraction.
Mr. Rutley stated that one of the felstone dj^kes in the Lake- district showed spurious cleavage in the direction of the dyke.
Mr. Murphy remarked that as the cooling commenced at the surface, the percolation of water between the first formed columns would aid in the production of the columnar structure lower down.
The Author, in reply to Mr. Judd, said that he did not regret having introduced the Auvergne phonolites into his paper, for he was convinced that their structure could not be explained by pressure ; he considered that' the intricacy of the arrangement shown by his diagrams negatived the supposition. Separation of the minerals, as described by Mr. Judd, would not necessarily be fatal to his theory ; but in the case of the Roche Sanadoire, he had examined a section to see if it existed, and had not found it.
