28 



As Dr. Thomson's address has already been published in extenso, 

 and issued to the members, it is unnecessary to insert an abstract 

 of it here. 



On Wednesday evening, 17th November, Professor James 

 Thomson, LL.D., read a paper on "The Jointed Prismatic Struc- 

 ture of the Giant's Causeway, and other Basaltic Rocks." He 

 stated that the opinions prevailing among geologists as to the 

 manner in which the jointed prismatic structure in basalts and 

 other igneous rocks has arisen, involve generally one or other ; 

 or a combination of both of the two following principles : — 1st. 

 Prismatic fracture by shrinkage in cooling, like the cracking which 

 may be observed in starch or mud, in drying: — 2nd. An assumed 

 spheroidal concretionary action of the lava or basalt in solidifying 

 from the molten state. He had accepted, as appearing to be a 

 part of the truth, the supposition of prismatic fracture by shrinkage 

 in cooling : and, about seven years ago, had offered a theory of 

 the origin of the jointed prismatic structure, involving primarily 

 that supposition ; and which had been suggested or indicated to 

 him by certain phenomena which he had observed in the stones of 

 the Giant's Causeway.* 



This theory, although having one fundamental supposition in 

 common to it and to some other views which had been previously 

 put forward, yet in other respects was altogether divergent from 

 any such previous views. Whatever other attempted explanations 

 of the jointed prismatic structure he had met with, indeed, appeared 

 to him to be plainly and decidedly untenable. All that he had 

 met with, whether founded on the supposition of prismatic fracture 

 by shrinkage, or of spheroidal concretionary action, or of both 

 together, appeared to him essentially to involve incongruous or 



* He had submitted this theory to the Belfast Natural History and Philo- 

 sophical Society, in a paper read on November 26th, 1862; and had shortly 

 afterwards brought the matter under the notice of the British Association, at the 

 Newcastle meeting, in 1863, and a brief account of the chief points is to be 

 found printed in the transactions for that year. 



