850 REPORT—1 885. 
endorsing with modification Professor Sedgwick’s crystalline theory, 
held that ‘ both cleavage and foliation are due to the parallel transmission 
of planes or waves of heat, awakening the molecular forces and determining 
their direction,’ a view differing from that of Dr. Darwin only in that it 
assigns the part of pressure to heat. A similar idea with respect to 
‘cleavage had been expressed by Sir John Herschel.! 
X. The Relation of Cleavage to Earth-movements. 
The theories discussed in the foregoing pages make the cleavage of 
rocks a result of lateral thrust operating throughout larger or smaller 
tracts of country, and the extreme stages of the structure, which involve 
mineralogical as well as lithological changes, a consequence of the intense 
stresses in the earth’s crust? to which mountain-systems owe their 
structure. Into the ultimate cause of these mechanical forces we are not 
called upon to enter;* but some of the resulting peculiarities in the 
arrangement of cleavage planes over an extended area require a passing 
notice. 
As regards the strike of cleavage, its regularity over considerable 
distances‘ and its parallelism to the main axes of disturbance of the district 
in which it oceurs, need not be further dilated upon; local exceptions to 
the latter rule have been noticed by various observers. As regards the 
cleavage-dip, no such simple laws can be laid down. The angle of dip 
may have any value, though the most perfect cleavage is usually inclined 
ata high angle. So far as observation at any one locality can discover, 
the dip of the cleavage is quite independent of that of the bedding. On 
traversing a district of slate-rocks in the direction across the cleavage the 
dip is observed to change very slowly and gradually; when an abrupt 
variation is noticed there is reason to suspect some disturbance posterior 
to the production of the cleavage-structure.® In fact, subsequent tilting 
and faulting of the rocks may affect not only the dip but to some extent 
also the direction of strike of the planes of cleavage ; and wherever in the 
preceding pages we have referred to the cleavage-strike and cleavage-dip, 
the original strike and dip of the cleavage-planes ought strictly to be 
understood. As another possible source of error in observing the dip of ~ 
cleavage-planes must be noticed the not infrequent surface-curvature ° of 
those planes in consequence of movements of the rocks near the surface 
of the ground, a phenomenon well seen in the valleys about Snowdon. 
Dr. Charles Darwin,” who made careful observations of cleavage and 
foliation over a large part of the South American coast, suggested as an 
explanation of the varying and opposite dips that the cleavage-surfaces— 
though to the eye appearing plane—may possibly be ‘ parts of large abrupt 
curves with their summits cut off and worn down.’ Mr. Sharpe,® fol- 
lowing out this line, endeavoured to trace ont ‘ systems of cleavage’ in 
} Lyell’s Student's Elements of Geology, p. 592, 2nd ed. 
® Rindenzusammenschub (Heim), Gebirgsdruck (Lehmann), Pression orogénétique 
(Barrois), &c. 
° For a discussion of this subject, see the concluding part of Heim’s Mechanismus 
der Gebirgsbildung, 1878, Basel. 
* Sedgwick, Jukes, Darwin, Phillips, &c., op. cit. 
5 Cf. Jukes, Manual of Geology, p. 271, ed. 1862. 
®° De la Beche, Geological Manual, p. 42. Darwin, Geol. Obs. in South America, 
p. 160 (1846). Jukes, Student’s Manual of Geology. 
* Geol. Obs. in South America, p. 146 (1846); 2nd ed. (1876), p. 446. 
§ Quart. Journ. Geol, Soc., vol. iii. p. 74 (1847). 
