852 REPORT— 1885. 
The last point we have to refer to is one of some importance, as raising 
a possible objection to the mechanical theory of cleavage : it is the relation 
in point of time between the earth-movements and the production of the 
cleavage-structure. It has been pointed out by Professor Sedgwick,! Pro- 
fessor Phillips,? Mr. Sharpe,’ and others, that the cleavage appears to 
have been impressed on the rocks subsequently to their being thrown 
into synclinal and anticlinal folds by the disturbing forces. Mr. Fisher,* 
insisting on this view, maintains that ‘cleavage is due to an internal 
movement of the rocks rendered necessary by the disturbed region being 
left, after elevation, in a position too lofty for equilibrium.’ This theory, 
it will be noticed, ascribes the elevation of the area in question to forces 
acting directly from below upwards. I have endeavoured elsewhere * to 
show that the kind of movement advocated by Mr. Fisher would result 
in an arrangement of the cleavage-planes and a difference in the perfec- 
tion of the structure in different parts of the area, which is quite out of 
accordance with the facts. 
It is evident that the cleavage-planes would be affected by so much of 
the disturbance of the rocks as was subsequent to their formation, and con- 
sequently the cleavage-structure, at least in its final state, must have been 
of later origin than the foldings by which it is not disturbed. Professor 
Tyndall ® seems to be of opinion that the production of the cleavage was 
more or less simultaneous with the disturbance, and consequently has 
been actually affected in a certain measure by the displacement of the 
rocks in which it occurs. At least this is apparently implied in his ex- 
planation, quoted above, of the deviations experienced by cleavage-planes 
in traversing alternating strata; this phenomenon admits, however, as we 
have seen, of a different explanation. It may be remarked too, that the 
fan-like arrangement which often characterises cleavage-planes in a dis- 
trict of disturbed strata might be connected with a certain amount of 
elevation in the central parts of the area subsequently to the setting up 
of the cleavage structure. But this does not in any way touch the 
fact that in any ordinary slate district the planes of cleavage may be seen 
ranging with approximate parallelism through contorted beds, the irre- 
gularities of which do not in any way affect the former. From this we 
cannot but infer that the impression of the cleavage structure on the 
rocks is an event of later date, in the main, than the tilting and flexures: 
observable in them. 
Granting this, however, we are still able to regard the cleavage and 
the folding as concomitant, though not simultaneous, effects of the same 
lateral pressure. As has been remarked by Mr. Fisher himself, they are 
two distinct ways of satisfying the same lateral compression of an area. 
But as the cleavage involves a rearrangement of the intimate parts of the 
rocks and an actual compression of their bulk, we might naturally expect 
it to be a later result of the lateral pressure than those changes which 
merely consist in displacement of the rock-masses as a whole. This con- 
sideration may fairly be held to remove the difficulty alluded to in accept- 
ing the theory which refers the origin of cleavage to the same mechanical 
stresses that brought about the disturbed position of the strata. 
1 Geol. Trans., 2nd ser. vol. iii. p. 474 (1835). 
2 Brit. Assoc. Rep., 1843, Trans. Sect., p. 60; 1856, p. 373. 
& Quart. Journ. Geol. Soc., vol. iii. p. 104 (1847). 
4 Gel Maq., 1884, pp. 397, 275, 276. 5 Tbid., 1885, p. 15. 5 Loe. cit. sup. 
