Correspondence — Mr. G. Poulett- Scrape. 625 



C O IR-IRIE S IPO ZtsTZDIE InTO IE . 



CUP-SHAPED JOINTS OF BASALTIC COLUMNS. 



Sir, — It is clear (from Mr. Mallet's letter to you, see Geol. Mag. 

 Nov. 1875, p. 566, and also from his communication to " Nature " 

 of this date) that no facts which may be adduced can be regarded as 

 of any value, if they discountenance a ' cut-and-dried ' theory on 

 which a ' physicist ' has made up his mind. He contents himself 

 with simply reasserting his theory, and resolutely refuses to examine 

 the appearances presented by the fine group of columns in the Hall 

 of the Geological Society, to which I have referred him, as being 

 totally inconsistent with it. 



Mr. Mallet's theory presupposed, in his own words, that "the 

 convex surface of the joint " should " always point in the same 

 direction as that from which the cooling and consequent splitting 

 proceeded" (p. 182 of Proceedings of Eoyal Society, No. 158). I 

 ventured to submit this supposition, which did not agree with my 

 experience, to the test of " facts." In the triple group of columns 

 from the Giant's Causeway in the possession of the Society, in which 

 there is every reason to suppose the cooling and splitting bad pro- 

 ceeded throughout in one and the same direction, do the convex sur- 

 faces of their joints all point in the same direction ? I found them, 

 on the contrary, pointing in different directions. Nay, even in one 

 column an articulation of little thickness showed^ two cup-shaped 

 concavities pointing different ways, back to back, like those of a 

 bi-concave lens. Now how does Mr. Mallet attempt to get over 

 this difficulty? Why, by supposing, or rather asserting as a fact 

 proved by Ms theory, that the cooling process in this column pro- 

 ceeded in opposite directions, from the top as well as the bottom, 

 and met in the interval between the two opposite concave joints — 

 that interval being an articulation only a few inches thick, and 

 showing no sign of seam or separation across it ! But, in addition 

 to the obvious improbability of this supposition, Mr. Mallet has 

 himself disposed of it in the following passage (page 183, Proc. Eoyal 

 Soc. No. 158) : " If the mass cools both from the top and the bottom, 

 the prisms, vertical and straight, will meet in an irregular interme- 

 diate stratum of angular fragments." 



I have already said that there is no appearance of any such inter- 

 mediate fragmentary stratum within the very thin articulation in 

 which Mr. Mallet, in order to save his theory, now chooses to place 

 the separating plane between the portions cooled from above and 

 from below. 



In addition to the evidence furnished by the column in the Society's 

 Museum, which, however, is quite conclusive on the question, I have 

 the authority of my friend Mr. Judd, whose competency as an ob- 

 server will not be disputed, for the fact that, in the platform of the 

 Giant's Causeway, as well as at Staffa, there are to be seen at least 

 as many concavities as convexities. And even Mr. Mallet will 

 scarcely deny that in all these columns the cooling must have pro- 

 ceeded in the same direction ; namely, from below upwards. Indeed 



