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. THe alleges “that cleaved limestones pos- 
sess no crystalline polarity,” and that in place of er. stallization producing slaty 
cleavage, it has a contrary tendency, and, when perfect and complete, obliterates 
it altogether. Mr Sorsy 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 Davin Forves,t 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.{ 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, o7 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 Smarre, Mr Sorsy, Mr 
Davip Forses, and other geologists, 1am 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 
* LyeLt, p. 612. ¢ 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. 
