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The chief points of his own theory may be briefly sketched out 

 as follows : — He supposes that the division into prisms has arisen 

 by splitting — through shrinkage — of a very homogeneous mass in 

 cooling : and that the cross-joints are fractures, which have com- 

 menced in the centre of the column, and have advanced to the 

 outside, as a circle increasing in diameter. This mode of fracture, 

 he thought, was evidenced by various markings and other indica- 

 tions on the stones. They usually show a remarkably symmetrical 

 conformation round the outer parts of their cross-joint faces, pre- 

 senting an appearance which had struck him as being like a com- 

 plete circular conchoidal fracture, often with roughly-figured rays 

 from the centre, such as in the ordinary conchoidal fracture are 

 seen emanating from the point where the blow has been struck. 



Now there are not many very distinct ways in which we can 

 suppose a fissure to have spread across a column or prism of solid 

 stone. First, if we for a moment suppose the fissure to have 



* The cross-joints he thus takes to be posterior to the prismatic fissures. But 

 according to the spheroidal concretionary theory they are supposed to be con- 

 temporaneous in origin with the prismatic faces of the columns, both the longi- 

 tudinal faces and the cross-joint faces being in that theory supposed to be 

 different pai;ts of the surfaces of spheroids growing larger in solidifying till they 

 meet, andean grow no more; or till they "press against each other" and 

 "squeeze" themselves together, so as to receive flattened faces, instead of a 

 rounded form. And according to the views of some who maintain the supposi- 

 tion of prismatic fissures by contraction, and deny the spheroidal concretionary 

 theory (Mr. Scrope, for instance — Volcanoes, 2nd ed., 1862, p. 104) the cross-joints 

 are supposed to be contemporaneous at each part of the length of the column, 

 with the prismatic fissures at that same place; the cross-joints being supposed 

 to be successive bounding faces between the solidified end of the column and 

 the as yet molten lava, into which the solidification is advancing, and the pris- 

 matic fissures being supposed at each period to extend quite forward to the 

 molten lava. Mr. Scrope's view, as he himself states in the passage re- 

 ferred to, comprises the supposition that the concavity of the ball-and-socket- 

 like cross-joints ought to be always directed upwards ; or, in other words, that 

 each separate piece of the jointed column ought, according to his supposition, 

 to have its bottom convex and its top concave. This supposition is not verified 

 but is decidedly controverted by the basaltic columns of the Giant's Causeway, 

 the cross-joints being often concave upwards, and often concave downwards, 

 and often nearly flat. 



