ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 841 
the same lateral pressure which, as a final result, produced the cleavage 
structure in the manner described in preceding sections. This supple- 
mentary hypothesis therefore must be regarded as an essential part of the 
mechanical theory of slaty cleavage. 
It appears from the above remarks that a system of close parallel 
planes of discontinuity in a rock, though lacking many of the charac- 
teristics of slaty cleavage proper, may be in general effect very similar 
to it; and, again, that such a structure may be itself a first step towards 
the true slaty cleavage. Such planes of discontinuity, however, are not 
properly speaking joints, but rather small faults. They are due to 
lateral pressure, and their distance apart is determined by the dimensions 
of the small folds which are a first step towards the faulting. True 
joints, on the other hand, although, as in the case of the ‘cleat’ of coal, 
they may be very close together, must be ascribed proximately to lateral 
tension, and their distance apart seems to be regulated by the lithological 
character of the rocks in which they occur,' and doubtless also by the 
thickness of the beds which they traverse. 
If, however, a rock having originally a true jointed structure be sub- 
sequently subjected to lateral pressure, it may fairly be supposed that 
this pre-existing jointing will affect the results of the pressure. Thus, 
unless the direction of the pressure be nearly perpendicular to the joints, 
the rock may conceivably begin to yield by sliding along the joint-planes, 
which thus become small reversed faults, and the subsequent effects of 
the pressure on the rock may be modified accordingly. This point is 
emphasised by Professor W. King,” who, however, makes it the starting- 
point of a general theory of slaty cleavage, in which he will probably 
have few followers. Stated in his own words, his thesis is that ‘ Slaty 
cleavage is essentially the result of pressure exerted against divisional 
planes, chiefly belonging to jointing, that existed in any given rock prior 
to its becoming affected by such pressure.’ But there seems to be no 
evidence to show that jointing or any other special structure is a neces- 
sary preliminary to cleavage, and it is yet to be proved that the direction 
of the cleavage-planes produced in a rock by lateral pressure is in any 
way dependent on those pre-existing planes of weakness, which we should 
naturally expect would be obliterated as such by the action of the 
pressure itself. 
In connection with the subject of jointing, it may be noted that many 
of the recorded instances of a second direction of cleavage in rocks are 
due to a confusion between cleavage and fine lamellar jointing.* 
VIII. Comparison of the Cleavage- and Fluxion-structures. 
We shall find it convenient to use the term fluxion- or flow-structure* 
in a wide sense, to embrace all those structures, whether macroscopic or 
! Phillips, Zllustrations of the Geology of Yorkshire, pt. 2 (1836); London. Fisher, 
Geol. Maq., 1884, p. 211. 
2 «Report on the Superinduced Divisional Structure of Rocks, called Jointing, and 
its Relation to Slaty Cleavage,’ Trans. Roy. Irish Acad., vol. xxv. Sci. p. 605 (1875). 
An Old Chapter of the Geological Record, &c., Appendix, p. 107 (1881). 
8 MacCulloch, Introduction to Geology, new ed. (1833). Sharpe, Quart. Jowrn. 
Geol. Soc., vol. v. p. 115 (1849). De la Beche, Geological Observer, pp. 712, 713 (1851). 
Nicol, Quart. Journ. Geol. Soc., vol. xv. p. 113 (1859). Davies, Slate and Slate 
Quarrying, pp. 25, 48, 49, 2nd ed. (1880), &c. 
1 Benegungs-Phiinomen (Weiss, 1866), Fluidal-Textur (Vogelsang, 1867), /luctua- 
tions-Textur (Zirkel, 1867). 
