production of the Prismatic Structure of Basalt. 123 



whether the prismatic jointing is to be attributed to the con- 

 traction by cooling of a previously melted mass cannot with 

 certainty be gathered, or whether the structure is due to pre- 

 existing concretionary or crystalline arrangement of the inte- 

 gral particles of the mass, or to this co-acting with enormous 

 external pressures, the origin of which is left perfectly vague, 

 or to some play of successive and joint actions of all these vari- 

 ous forces, the writer has found himself unable clearly to gather. 

 Mr. Scrope, who has written somewhat at length on the subject 

 (' Volcanoes/ second edit. p. 102, 1872), says, "It appears to 

 me that the explanation of this columnar structure given in 

 most text-books is scarcely complete or satisfactory. M. Delesse, 

 indeed, holds it, as 1 do, to be the combined result of crystalli- 

 zation (rather I would say concretionary attraction) and con- 

 traction," i. e. by cooling ; and it will be apparent upon collating 

 chapter vi. pages 93-109, that the author considers the play of 

 crystalline or concretionary forces, as they are vaguely termed, 

 essential elements to the production of columnar structure 

 (which is incapable of being produced by contractile forces alone) ? 

 and does not view these concretionary or crystalline forces as 

 merely modifying conditions acting subsequently to the produc- 

 tion of columnar and jointed structure produced by contraction 

 alone in the particles of the mass. The views here referred to 

 differ but little from those expressed by Sir Henry De la Beche 

 in his ' Geological Observer ' of 1853, chap. xx. 



Professor James Thompson, now of the University of Glasgow, 

 communicated a paper to the Geological Section of the British 

 Association in 1863 (an abstract of which appears in Brit. Assoc. 

 Report, 1863, Proc. of Sections, p. 89), in which he develops 

 some new views on this subject. 



After the lapse of eleven years he has in substance repeated 

 those views in a paper on the Giant's Causeway,read before the 

 late Meeting of the British Association at Belfast (Belfast c News 

 Letter/ August 27, 1874). Professor Thompson has done ser- 

 vice to science by correcting some erroneous statements as to 

 matters of fact, and by showing that concretionary or crystalline 

 forces have probably had nothing to do with the production of 

 prismatic and jointed structure. He merely accepts the views 

 of previous authors as to the production of the prisms by the 

 contraction of the mass; and in that part of the paper which 

 alone appears to the writer to possess claim to novelty, viz. his 

 explanation of the production of the cup-formed cross joints, his 

 own views appear to the writer untenable. 



In the present paper the writer proposes to treat of the 

 production of columnar and jointed structure in a somewhat 

 more determinate manner than has, so far as his knowledge 



