Ixii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



vertical foliation, represents it as fan-shaped, and explains it upon 

 his own principles ; and that M. Favre has not been able to detect 

 the uniformity of cleavage over large spaces which Mr. Sharpe 

 describes. Such discrepancies as these may well occur in a region 

 like the Alps, which cannot be explored, as Professor Forbes so 

 justly observes, in a few days, but requires the conjoint exercise of 

 high physical and mental powers continued for many years. 



Passing now to the subject of Cleavage, which I have reserved to 

 the last, I necessarily refer first to Professor Sedgwick, who, in his 

 description of the rocks of Cumberland and Wales, read to our 

 Society in 1835, entered into the general theory of the structure 

 of large mineral masses, and necessarily into that of cleavage, the 

 planes of which he clearly distinguished from those both of stratifi- 

 cation and of joints. The highly fissile rocks he was then exami- 

 ning naturally led him to explain this peculiarity of structure by the 

 theory of crystallization ; and, although such rocks cannot be looked 

 upon as large crystals, an idea which does not appear to me to have 

 been ever entertained by Professor Sedgwick, there is certainly an 

 analogy between the planes of rocky and of mineral cleavage, as the 

 mechanical pressure which has produced the one is replaced by the 

 force of affinity in the other, the cleavage-planes of a crystal indi- 

 cating the directions of the attractive forces, just as those of rocky 

 cleavage do the direction of pressure. 



Mr. Sharpe takes his starting-point from this paper, though with- 

 out admitting the principle of a true crystallization. His first step — 

 and the thought was a happy one — was to connect the distortion of 

 organic remains with the cleavage of the slaty rocks containing them. 

 Professor Phillips had, as stated by Mr. Sharpe, before pointed out the 

 distortions of both shells and trilobites, and ascribed them to a 

 creeping movement of the particles of the rock along the planes of 

 cleavage ; but Mr. Sharpe shows, from many examples, that the 

 distortion of shells is the result of the two conditions of any com- 

 pressible body under pressure, namely, expansion in one and com- 

 pression in the other direction, and by combining this with other phee- 

 nomena, he arrived at the conclusion that the direction of the pressure 

 producing contortions is perpendicular to the planes of cleavage. 

 This connexion of the cause of contortion with that of cleavage is the 

 chief merit of the paper ; and the objection of Professor Rogers, that, 

 if the contortions be merely the mechanical effects of pressure, they 

 differ in character from the molecular rearrangements of particles 

 necessarily attendant upon cleavage, may be readily answered by 

 stating that, whilst a body yields to pressure, the molecular action 

 does not commence, but, when the limit of condensation and expan- 

 sion has been attained, the molecular action will commence and the 

 cleavage-planes be produced. In a similar manner, the objection of 

 Professor Rogers founded on the regularity of cleavage-planes when 

 passing through contorted rocks — namely, that as a parallel force 

 acting on curved surfaces must necessarily meet that curve at 

 different angles and therefore could not give rise to a uniform 

 cleavage, — may be answered ; for, whilst the beds admit of yielding, 



