460 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



rectly as the amount of mechanical compression to which they have been sub- 

 jected, and that this compression was such as would necessarily change the struc- 

 ture of uncleaved into cleaved rock. He alleges " that cleaved limestones pos- 

 sess no crystalline polarity," and that in place of ciystallization producing slaty 

 cleavage, it has a contrary tendency, and, when perfect and complete, obliterates 

 it altogether. Mr Sorby conceives that the absolute condensation of the slate 

 rocks amounts, upon an average, to about one-half of their original volume* This 

 condensation he ascribes to the forcing together of the particles, and the filling 

 up of their interstices by pressure perpendicular to the cleavage, and partly by 

 elongation in the direction of the cleavage dip. 



Mr David Forbes,| writing upon foliation in rocks, leans to the conclusion 

 that foliation is a distinct phenomenon from cleavage, and that the causes pro- 

 ducing them were also distinct. He refers the foliation to chemical action, the 

 cleavage to mechanical pressure. He admits that the planes of foliation and 

 those of cleavage are often parallel to one another 4 But the parallelism of the 

 foliation to the cleavage he ascribes to a previously induced cleavage structure 

 facilitating crystalline lamination in its own planes. 



He supposes foliation to have resulted from a chemical action combined with a 

 simultaneous arranging molecular force, developed at heats below the semifusion 

 of the mass ; also that the arrangement of foliation is often due to the proximity 

 of igneous rocks, and tends to follow the direction of any lines in the rocks where 

 the cleavage stratification, or strice of fusion, follow preferably those lines offering 

 least resistance. 



Examination of the Prevailing Theories of Cleavage and Foliation. 



From the theory of the origin of cleavage by mechanical compression exerted 

 perpendicularly to the cleavage planes, as adopted by Mr Sharpe, Mr Sorby, Mr 

 David Forbes, and other geologists, I am constrained to dissent, and upon the 

 following grounds : — 



1. It has been already shown, in the general description of the phenomena of 

 cleavage, that this tendency of fissuration is stronger and weaker in alternate 

 closely contiguous planes, and is not diffused equally, even in the one direction, 

 through the mass. Now it is impossible to conceive how a purely mechanical 

 compression could have occasioned a regular alternation of greater and less con- 

 densation of particles, all equally free to move and adjust themselves into posi- 

 tions of statical equilibrium, and all equally subjected to the same amount of 

 force. The well-known law of a quaquaversal tension of fluids is manifestly ap- 

 plicable to partially soft and flexible rocky matter, if we are to impute to this an 



* Lyell, p. 612. f See his Paper, Quarterly Journal, Geological Society, 1855. 



+ See his Paper for a good figure of deflection of cleavage and foliation in the margin of a vein of 

 quartz. 



