816 REPORT—1885. 
and polar forces acting on the whole mass simultaneously, in given direc- 
tions, and with adequate power.’ Sir H. De la Beche! invoked the 
agency of electric currents and terrestrial magnetism, and cited in con- 
firmation of this theory the experiments of Messrs. R. W. Fox? and 
R. Hunt. Mr. Hopkins,‘ too, endeavoured to show a connection 
between cleavage and ‘ magnetic currents.’ Dr. Charles Darwin,® whose 
views will be further referred to, was led to suspect ‘ 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 
consolidation of the mass and the total cessation of all molecular move- 
ment.’ The mechanical theory gained ground slowly, geologists being 
apparently reluctant to admit so simple an explanation of the phenomena. 
Mr. J. Beete Jukes,® for example, generalising from his observations in 
Newfoundland, wrote thus: ‘ The same causes which gave their prevalent 
and general direction to the mechanical forces by which the rocks were 
elevated from their original position, and their strike and dip produced, 
likewise determined the direction in which those forces should act (what- 
ever they were), which produced the cleavage.’ Mr. Sharpe’ in his first 
paper on slaty cleavage, while admitting the action of pressure in the 
production of cleavage, declined to regard it as the sole agent concerned, 
and considered that heat may have had some share in the process; in his. 
second paper, however, he had apparently arrived at a purely mechanical 
theory. But it still remained for Mr. Sorby* to show that all the main 
facts connected with slaty cleavage are explicable as the effects of a dis- 
tortion of the mass of the rock consequent upon lateral compression. 
This theory was strengthened by the synthetic experiments of Mr. Sorby 
and Professor Tyndall,° in which a fissile structure resembling cleavage 
was artificially produced in various plastic substances by the agency of 
lateral pressure alone. 
Il. The Mechanical Theory of Slaty Cleavage : the Distortion of 
Cleaved Rocks. 
It will be convenient at this point to put forward briefly the views of 
those geologists who have offered explanations of slaty cleavage founded 
solely on mechanical principles. These theories, however complete in 
themselves, are to be regarded not as the end of the investigation but 
rather as important landmarks by the way ; for they are applicable only 
to the slaty cleavage structure proper in its various degrees of perfection, 
and there may even appear reasons for doubting whether the whole trath, 
even as regards roofing-slates and cleaved limestones, can always be 
expressed by a purely mechanical theory, in the sense implied here. 
1 Report on the Geology of Cornwall, Sc. p. 281 (1839), Kc. 
2 Reports of Cornwall Polytechnic Soc., 1837, pp. 20, 21, 68, 69. Of. Phillips, 
Treatise on Geology, vol. ii. p. 87 (1839). 
3 Mem. Geol. Surv. Gr. Brit., vol. i. p. 433 (1846). 
4 The Connection of Geology with Terrestrial Magnetism, ch. xiii. (1848). 
5 Geological Observations in South America, p. 168 (1846). 
© Excursions in Newfoundland, vol. ii. p. 325 (1842). 
7 Quart. Journ. Geol. Soc., vol. iii. p. 104 (1847). Ibid. vol. v. p. 129 (1849). 
8 Edinb. New Phil. Journ., vol. lv. p. 137 (1853). Phil. Mag., 4th ser., vol. xi. p. 20, 
and vol. xii. p. 127 (1856). 
® Phil. Mag., 4th ser., vol. xii. p. 37 (1856). 
