WORKS ON GEOLOGY, MINERALOGY, AND PALHONTOLOGY. 813 
(495) Crossxey, [Rev.] H. W., and C. J. Woopwarp. The Post 
Tertiary Beds of the Midland District. Ibid. p. 42. 
(496) The Birmingham Saturday Half-Holiday Guide. Fossils by 
S. Autrorr. Geology by C. J. Woopwarp. 8vo. Birmingham. 
On Slaty Cleavage and Allied Rock-Structures, with special 
reference to the Mechanical Theories of their Origin. By ALFRED 
Harker, M.A., F.G.S. 
[A communication ordered by the General Committee to be printed in extenso 
among the Reports. | 
I. ‘Introductory and Historical. 
Tue following paper is intended to deal in a comprehensive, though far 
from exhaustive, manner with the main phenomena of the cleavage 
structure in slate-rock, the received views of the origin and significance 
of these phenomena, and the relations existing between slaty cleavage 
and various allied rock-structures. It may be regarded as to some extent 
supplementary to the Report on Cleavage and Foliation presented to the 
British Association at the Cheltenham meeting in 1856 by the late Pro- 
fessor John Phillips;! and accordingly a few words seem to be called for 
in justification. 
The facts so ably summarised by Professor Phillips were mnch less 
complete than those now at our disposal, and although they were perhaps 
sufficient in 1856 to establish on a firm basis the mechanical theories of slaty 
cleavage which had been already put forward by Messrs. Sharpe and Sorby, 
it could not be said that such theories had obtained anything like universal 
acceptance. Indeed Professor Phillips had in contemplation? a further 
report, in which the ‘mechanical pressure’ which these authors advo- 
cate’ was to be ‘placed in comparison with the crystalline polarity, 
formerly advanced by Professor Sedgwick,’ and in which ‘Mr. Fox’s 
ingenious imitation of slaty cleavage by electrical currents’ was also to 
receive attention. Following the tendency of more recent speculations, 
I propose to discuss the subject of slaty cleavage from a purely mechani- 
eal standpoint. Such an investigation will involve considering the rela- 
tions of cleavage to contortion, to faulting, to jointing, and to foliation, 
subjects which had received but little notice at the date of Professor Phillips” 
report, but some of which are now recognised as among the foremost 
questions of geology. 
Slaty cleavage may be defined as a superinduced tendency in a rock 
to split in a definite direction more readily than in other directions, such 
direction of splitting being independent of the planes of deposition in a 
sedimentary rock. In the classical paper of Professor Sedgwick,’ referred 
to below, the following passage occurs : ‘ Besides the planes of cleavage, 
we may often find in large slate quarries one or more sets of cross joints, 
which, combined with the cleavage, divide the rock into rhombohedral 
solids. Should any one assert that this subdivision of slate rocks into. 
rhombohedral solids implies three planes of cleavage, we might reply,. 
1 British Association Report, 1856, p. 369. ‘ Report on Cleavage and Foliation in, 
Rocks, and on the Theoretical Explanations of these Phenomena. Part I.’ 
? Op. cit., p. 392. * Geol Trans., 2nd ser., vol. iii. p. 473, 1835. 
