104 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 2, 



planes ; and that the joints bearing from N.N.W. to S.S.E. are se- 

 condary fissures at right angles thereto. The variations observed at 

 different places both in the direction of the joints and in the strike 

 of the cleavage, make it probable that the axis of elevation will be 

 found not a straight line, but slightly bent, d fjd oi r 



gs b9vi909i 9d fiBD 1)9' Conclusion. 



Throughout all the preceding pages I have abstained from ex- 

 pressing any opinion of the cause producing the cleavage ; and now 

 that I have gone through the subject, I must still leave the imme- 

 diate agent in the operation undiscovered ; although I hope that its 

 discovery may be facilitated by the progress made in ascertaining the 

 circumstances under which it took place. 



Pressure appears to have been concerned in the operation; for 

 the cleavage is uniformly at right angles to the direction in which 

 pressure is seen to have taken place ; and also the amount of clea- 

 vage appears to bear some proportion to the compression suffered by 

 the rock. On the other hand, there are reasons for thinking that 

 pressure could not be the sole agent in the operation, for the clea- 

 vage did not take place on the first upheaval of the district, when 

 the crust not having yet given way the pressure might be supposed 

 the greatest, but only after the beds had assumed their present posi- 

 tion and the various anticlinal and synclinal axes had been formed. 



Heat may have had some share in producing the cleavage : if the 

 elevation was caused by a heated mass below, the conduction of heat 

 must have followed the same direction as the pressure; and each 

 sheet of slate must from its position have received the heat sooner 

 than the sheet above it while the temperature was increasing, and 

 parted with it later while the mass was cooling. 



Galvanism has been supposed to have caused the cleavage of slate 

 rocks, and the experiments of Mr. R. W. Fox*, since repeated and 

 extended by Mr. R. Huntf, have been brought forward in proof of 

 its agency, and the fact that lamination has actually been produced 

 in clay by galvanic action tells much in its favour ; but before it can 

 be admitted to have produced the cleavage, it ought to be shown 

 that the circumstances of the case were such as would have pro- 

 duced galvanic action, and that it would have acted in the direction 

 required. 



Lastly, Mr. Darwin has suggested an explanation built upon a com- 

 bination of mechanical and crystalline forces, viz. " that the planes 

 of cleavage and foliation are intimately connected with the planes of 

 different tension to which the area was long subjected, after the main 

 fissures or axes of upheavement had been formed, but before the final 

 cessation of all molecular movement:};." And that " this difference in 

 the tension might affect the crystalline and concretionary processes §." 



These seem to be the agencies among which we have to seek, 



* Report of Cornwall Polytechnic Society, 1837. 



t Memoirs of the Geological Survey of Great Britain, vol. i. p. 433. 



X Geological Observations on South America, p. 168. § Ibid. p. 167. 



