470 Mr. HOPKINS, ON THE INTERNAL PRESSURE OF ROCK MASSES. 



experiments made in the fiist instance by Mr. Fox, and afterwards repeated by Mr. Hunt, as 

 described by the latter gentleman in an interesting memoir, contained in the Memoirs of the Geo- 

 logical Survey of Great Britain, Vol. i., on the influence of magnetism on crystallization. It 

 would be foreign to my object to enter into any discussion of a theory founded on these experi- 

 mental results ; and indeed detailed discussion of any theory on the subject would be, I conceive, 

 at present entirely premature ; but I would remark that views of the subject founded on these 

 results, and those founded on the observed facts respecting the distortion of organic forms, are 

 by no means to be considered as opposed to each other. On the contrary, it is very possible 

 tion we may hereafter be better able to account for the phenomena of lamination by the joint opera- 

 that of the causes to which they would be referred respectively according to these two views of 

 the subject, than by the independent operation of one of those causes only. 



In concluding this communication, I would especially remark that the advocacy of any parti- 

 cular theory on the subject of cleavage structure has formed no part of my object. Our ignorance 

 of the physical causes of crystallization, or the manner in which such supposed causes may ope- 

 rate, is too great to admit of our forming at present any theory on the subject which might not be 

 deemed altogether premature. All I would here insist upon is this — that the facts observed by 

 Professor Phillips and Mr. Sharpe indicate certain determinate relations between the distortions of 

 organic forms and the positions of the planes of lamination of the beds in which those forms are 

 discovered, relations which seem to imply that the forces which produced the distortion had also 

 their influence in determining the planes of lamination. My object has been to point out the 

 accurate mechanical conditions of the problem, and thus to indicate the points to which the 

 attention of future observers should be especially directed in order that their observations may 

 afford conclusive tests of the truth of any theory which may hereafter recognize the efficiency of 

 the mechanical agency explained in this paper, as one of the causes of the laminated structure. 



W. HOPKINS. 



C-AMBRiDQE, Moy S, 1847. 



