AM) SPHEROIDAL BTB.TTCTTJBE. 153 



metry in the surface of rupture ; and thus surfaces of revolution 

 (such as spheroids, hyperboloids, &e.) would be generally produced. 



There is thus a certain relation between the spheroids, the curved 

 horizontal joints, and the great curving cross joints, such as are seen 

 at the Koche Tuilliere. Hence the fissile structure so well exhibited 

 both here and at the Eoche Sanadoire is not a cleavac/e structure in 

 the ordinary * sense of the word ; for it is the result of force acting 

 in exactly the opposite direction, being the result of tension ; whilst 

 cleavage is due to compression — somewhat similar results being 

 thus produced by forces with changed algebraic signs. 



I consider the cup-and-ball joint structure a special case of the 

 spheroidal ; and the only point on which I feel some doubt is whether 

 one should regard the division as beginning at the centre or at the 

 exterior, or as to some extent independent, — namely, whether as a 

 column cools tolerably uniformly, it tends by symmetry to divide into 

 approximately equal lengths, and so spheroids are formed in the more 

 plastic though warmer parts (the exterior shell being a little more 

 rapidly chilled), and whether the cracks thus formed between two 

 adjacent spheroids are continued toward the exterior, which then, 

 being in a state of strain, is cracked horizontally at these points of 

 weakness ; or whether the exterior, cracking first, caused a line of 

 weakness, which determined the commencement of a spheroid in the 

 inner parts ; or, as a third possibility, whether the two divisional 

 surfaces are to some extent independent — the spheroids forming 

 within in the more plastic part of the column, the cracks opening 

 from without to the more solidified part, and the two surfaces of 

 division running together at last so as to complete the separation. 



Some independence in these surfaces of division seems to be indi- 

 cated by the occurrence of distinctly formed spheroids in an unbroken 

 column, and by the fact that the spheroids are occasionally again 

 subdivided into segments which to some extent continue the same 

 structure, two or even more of these being enclosed in a more 

 regular spheroidal shell (see fig. 13). This independence, too, 

 would explain the fact that the ends of columns pointing in the same 

 direction show sometimes cups and sometimes balls. It is quite true 

 that the divisional curves, if due to strain from contraction, should 

 be concave to the surface of cooling, as Mr. Mallet has proved (and 

 this, I have little doubt, is the case in the curvitabular structure) ; 

 but, as Mr. Scrope objects, and as my own experience has shown 

 me, in the case of cup-and-ball structure in columns there is great 

 uncertainty, adjacent columns showing at top, one a cup, the other a 

 ball. If, however, the two fissures were formed to some extent inde- 

 pendently (the curved one beginning at the interior, the plane one 

 at the exterior), this would be likely to happen, though still the one 

 or the other structure might predominate : the outside cracks would 



* That it cannot be a true cleavage is shown (1) by the extreme improbability 

 of this being produced in rocks which are not likely to have been subjected to 

 great earth-movements and must have always been of rather a superficial cha- 

 racter, and (2), in the cases quoted, by the impossibility of explaining the ar- 

 rangement of the divisional planes on any theory of cleavage. 



