ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixiii 



as they must do in the process of contortion, the molecular action 

 will be slight ; but, when the yielding has ceased, there will be either 

 a force of tension or one of compression, as in an arch, and the re- 

 sulting cleavages take place in radiating planes ; or, should the whole 

 contorted strata form one mass, the cleavages would take place uni- 

 formly through that mass without reference to the contortions. 

 These objections are urged by Professor Rogers in an able paper read 

 before the Royal Society of Edinburgh, in which he repeats his 

 former views respecting the folding of strata as a consequence of the 

 supposed wave-like motion of an internal liquid mass, and states that 

 the cleavages are always parallel to planes passing through the anti- 

 clinal and synclinal axes of the waves. You are also all aware of 

 the ingenious observations of Mr. Sorby, ascribing cleavage in part 

 to the parallel arrangement of planes of mica and other uniaxial 

 particles within the mass ; and assuredly, so long as a mass is 

 capable of condensation, the planes of mica, like the fossils, must 

 fall into planes perpendicular to the direction of pressure, or, 

 if acted upon by a current whilst they are being deposited, 

 will tend towards such planes ; in a recent paper, Mr. Sorby 

 states that it was a misconception of his views to suppose that he 

 ascribed cleavage solely to the presence of plates of mica, as he 

 had recognized it in rocks not containing mica, though he con- 

 siders the presence of mica as tending to produce a much more 

 perfect cleavage. Professor Tyndall, by a series of interesting 

 experiments, has shown that cleavage can be produced in any sub- 

 stance by pressure, and that the planes of cleavage will be per- 

 pendicular to the direction of that pressure. Since then a most 

 interesting work has been published by Hausmann of Gottingen, in 

 which he collects together numerous examples of the molecular 

 rearrangement of solid bodies under the influence of continued 

 heat, as, for example, in bar-iron, where a granular structure is 

 changed into a laminated one, without any change of the external 

 form ; and, when we consider that in most geological phaenomena 

 heat must have been combined with pressure, we may well believe 

 that whilst heat facilitated the development of molecular re- 

 arrangement, pressure determined its direction. Such indeed 

 appears to be the necessary result of the great physical law of 

 action and reaction, as the particles, when thus pressed together, 

 necessarily react in an opposite direction, and thus a force of reaction 

 is produced sufficient to overcome the ordinary cohesive force of the 

 body : Professor Rogers' objection, that such cleavage ought to be 

 exhibited in coarse sandy rocks, as well as in those of finer materials, 

 whereas it is observed to miss the coarse beds in an alternating 

 series of coarse and fine strata, not being valid, as cohesive force 

 must also be in action to hold the particles together in order that 

 any satisfactory cleavage should be exhibited, and the defect of 

 cohesion in sand prevents therefore the development of cleavage. 

 The experimental proofs which have been adduced of the true origin 

 of cleavage have recently been supported by an interesting mathe- 

 matical demonstration, by the Rev. Professor Haughton, of the result 



