6 Dr. Boase's Remarks on Mr. Hopkins's 



the modifying circumstances, the possible existence of which 

 Mr. Hopkins has admitted, and stated that they would render it 

 very difficult to calculate with any precision the resulting phae- 

 nomena. One of the most influential of these circumstances 

 is the jointed structure in the mass subjected to the action 

 of the elevatory power. Mr. Hopkins, duly impressed with 

 its importance, has made many interesting remarks thereon, 

 more particularly concei'ning the coincidence of lines of struc- 

 ture and of fissure. 



The structure of rocks is a subject which has engaged my 

 attention for many years ; and in the seventh volume of this 

 Journal (p. 376 et seq.) I endeavoured to show that the joints 

 have not been mechanically produced, as Prof. Sedgwick sup- 

 poses, but have resulted from an arrangement of the particles 

 by the attraction of cohesion during the process of consolida- 

 tion, being in its nature similar to crystallization. The Pro- 

 fessor has, indeed, referred to Mr. Hopkins's " Researches" 

 in support of his views, but it will be seen by the following 

 passage that Mr. Hopkins, on physical grounds, has arrived at 

 the same conclusion as myself. " It has been shown that ex- 

 traneous forces could only tend to produce systems of fissures 

 crossing each other at right angles, whereas regular systems 

 of joints appear to meet each other frequently at acute angles, 

 and consequently must necessarily have been owing to some 

 different cause. I do not therefore conceive that any general 

 tension of the mass produced by extension from elevation, or 

 contraction in the course of solidification, can have had any 

 material effect on the formation of joints. It is probably, 

 I think, to be referred entirely to some kind of internal mole- 

 cular action." 



That the lines of structure are perfectly independent of any 

 elevatory power would seem to be implied by their occurring 

 even inclined in opposite directions entirely within horizontal 

 strata ; and also by their existence in solid rocks between beds 

 of incoherent substances, as, for instance, in the oolite of Buck- 

 land Point, in the parish of Mells, which Mr. Townsend has 

 described as rhombohedral beds dipping at an angle of 40°, 

 and confined between two horizontal beds of clay*. 



If then solid rocks have necessarily a jointed structure, one 

 of the data on which Mr. Hopkins's calculations are founded 

 is invalidated, in as much as the elevatory force can never 

 have acted on a solid mass without the interference of this 

 modifying circumstance. The abstract consideration of the 

 cjuestion I can easily conceive to be required in order to ar- 



* Grcenoiigh's Geological Essays, p. J3. 



