122 Mr. R. Mallet on the Origin and Mechanism of 



eliminated by the above-mentioned principles ; for their applica- 

 tion gives the equation \J$(a + b\/ — c) = V & (a-]-b v S — c). 



The further consideration of the meaning of */ — 1 in unita- 

 tion is deferred until geometrical relations are explicitly treated of. 



The peculiarities and practical uses of unitates are due in a 

 great measure to their being periodic functions. 



74 Brecknock Road, N. 

 July 1875. 



XVI. On the Origin and Mechanism of production of the Prismatic 

 (or columnar) Structure of Basalt. By Robert Mallet, 

 F.R.S.* 



I^TO branch of physical geology is superior in interest or in 

 J-^l the importance of its relations than that which regards the 

 jointing, lamination, and cleavage of rock-masses. 



Lamination and cleavage have been ably elucidated by Sharpe 

 and others. Our knowledge of the mechanism of the joint- 

 ing, though contributed to by Haughton, Phillips, and other 

 observers, is still very far from complete. Least complete of 

 all, however, is our knowledge of that which constitutes the 

 most singular of all observed forms of jointing, namely that 

 which has resulted in the production of that complicated system 

 of joints evidenced in columnar basalt. Even before the con- 

 troversy of the last century as to the igneous or aqueous origin 

 of basalt was settled (and the conclusion admitted very gene- 

 rally that basalt is an igneous rock differing from other lavas 

 chiefly in the conditions under which it was poured forth), the 

 circumstances attending the production of such Assuring as ends 

 in columnar structure was speculated on, but in the same 

 superficial and inexact manner that has generally characterized 

 the treatment of the subject by subsequent writers up to the 

 present time. So far as the writer's acquaintance with the 

 systematic works of British geologists and vulcanologists ex- 

 tends, including those of Bakewell, De la Beche, Lyell, Dau- 

 beny, Jukes, and several others, no consistent or even clearly 

 intelligible theory of the production of columnar structure can 

 be found. Even agreement upon the fundamental fact of 



* An abstract of the following paper was read to the Royal Society, and 

 will be found in its Proceedings (No. 158), January 21, 1875. After five 

 months had elapsed, and the paper, as I am informed, had been submitted 

 to four referees in succession, 1 received notice that " it was not deemed 

 expedient to print it at present." I offer no comment upon a decision as 

 to the grounds for which I am necessarily ignorant; but being unable to 

 find any radical error vitiating the general views of my paper, I venture to 

 publish it in its present form. 



