CXXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



but be received by the Society as a striking exemplification of the 

 zeal, originality, and success with which he carries on his researches 

 in physical geology. 



Cleavage. — In my last address I was necessarily obliged to enter 

 at considerable length upon the subject of cleavage, in order to do 

 justice to the labours of our late most distinguished and most lamented 

 President, and I shall therefore refrain from anything more than a 

 very brief reference to what has since then come under my notice. 

 The report upon cleavage presented to the British Association in 1856, 

 or anterior to my address, but published subsequently, has been 

 divided into two sections : the first, of a historical and descriptive 

 character, is that which has now appeared ; the second, which will 

 be theoretical, is reserved for a future period. With his usual ability 

 Professor Phillips has corrected some errors of detail in the observa- 

 tions of his predecessors; and it may be said that he clearly establishes 

 the following phases in the progress of a subject which he justly 

 characterizes as English ; viz., 1st, the recognition of the necessity 

 of a great leading or general cause, instead of a partial or accidental 

 one, by Professor Sedgwick ; 2nd, the more perfect connexion of 

 cleavage with pressure, by the methodical application of the phae- 

 nomena of distorted shells to its explanation by Mr. D. Sharpe; 

 3rd, the illustration of many points connected with cleavage, by the 

 microscopical investigations of Mr. Sorby; 4th, the experimental 

 investigation of the effects of pressure in producing cleavage, by 

 Professor Tyndall. The " fundamental generalization" of Professor 

 Sedgwick, to use the expression of Professor Phillips, has been illus- 

 trated by many observers, and especially by the two brothers Eogers 

 in their graphic account of the Appalachian chain in the United 

 States ; and I will only add, that whilst there can be no doubt that 

 cleavage has, in many of the cases which came under the observation 

 of Mr. Sharpe, been the result of pressure, it still remains to prove 

 that pressure has been the only cause of cleavage — or at least that 

 pressure has always acted in the same manner, in order to bring 

 within the scope of calculation some of the examples cited by Pro- 

 fessor Phillips, such as " waved cleavage," " cleavage varying in its 

 angle according to the variation in the mineral composition of suc- 

 cessive strata," &c. In my last address I gave my opinion as to the 

 natural law upon which cleavage might be supposed to depend; and I 

 must still say, that when Professor Sedgwick expressed the idea that 

 the cleavage in mountain-masses is so regular as to appear like the 

 results of enormous crystallization, he seems to have exhibited that 

 keen appreciation of natural phenomena for which he has always 

 been remarkable, and to have had in his mind the fact that crystals, 

 being formed by the apposition of successive layers of molecules drawn 

 together by the force of affinity, subsequently cleave in the direction 

 of these layers — the cohesive force between the molecules of any one 

 layer being greater than between the molecules of two adjacent layers. 

 At the last meeting of the British Association, Professor Haughton 

 illustrated his preceding calculation of the effect of pressure in dis- 

 torting organic remains, by an ingenious model, which showed that 



