836 REPORT— 1885. 
VII. Spurious and Incipient Cleavages. 
In a paper read before the British Association in 1857, Dr. Sorby! 
put forward what may be regarded as a limitation of his original theory 
of slaty cleavage. He showed that besides the structure contemplated in 
his earlier papers, which is ‘ quite independent of any actual fractures or 
breaks of continuity, and may be called wltimate-structure-cleavage,’ there 
is also ‘ a cleavage due to very close joints, often so close as to be quite 
undistinguishable unless a thin section is examined with the microscope, 
whilst the arrangement of the particles in the spaces between them is 
independent of the direction of the joints, and is often related to quite 
another plane. This kind of cleavage may therefore be called close-joints- 
cleavage. The distinction thus enforced is abundantly confirmed by the 
microscopic study of various slate-rocks, and we shall therefore find it 
convenient to use the term cleavage in a sufficiently wide sense to include 
not only the structure we have discussed above under the name slaty 
cleavage proper, but also other structures, which, though effectively 
identical with it, have arisen in a different manner. 
In fact Professor Heim, in his comprehensive work ‘ Ueber den 
Mechanismus der Gebirgsbildung,’ ? distinguishes three types of cleavage, 
which, however, may, and frequently do, occur in conjunction : 
(i.) Cleavage produced by a succession of displacements or minute 
faults, resulting from small contortions ; this he calls Ausweichungselivage. 
(ii.) Cleavage produced by the individual particles of the rock being 
flattened or elongated perpendicularly to the direction of maximum 
pressure ; this is Mikroclivage. 
(iii.) Cleavage produced by a parallel arrangement of all the flat and 
elongated constituents of the rock; to complete the terminology this 
might appropriately be styled Flwidaltexturclivage. 
This last type of cleavage is that originally described by Dr. Sorby ; the 
Mikroclivage, characteristic of limestones and similar rigid rocks, is that 
more particularly emphasised by Mr. Sharpe; while the Awsweichungs- 
clivage is the structure, or rather the set of allied structures, which we 
have now to discuss, and which includes the ‘ close-joint-cleavage’ 
mentioned above. It includes also those structures in which parallel 
planes of weakness (not actual discontinuity) occur in a rock at certain 
finite distances apart. If there be actual surfaces of disruption, there 
may or may not be appreciable displacement along them; the former case 
presents a series of minute reversed faults of very steep bade. They are, 
of course, ‘ close,’ as distinguished from open joints, and they may subse- 
quently become sealed up, either by mere cementation or by actual 
foliation taking place along them. If they thus become obsolete as 
surfaces of weakness in the rock, there may even be produced by subse- 
quent changes asecond cleavage cutting across the first, a result which 
we have seen to be impossible with true slaty cleavage. 
Ausweichungsclivage assumes locally many curious forms, and these 
structures are met with on very various scales of magnitude, being some- 
times very minute, at other times readily detected by the naked eye, when 
* ‘On Some Facts connected with Slaty Cleavage,’ Brit. Assoc. Rep., 1857, Trans. 
Sect. p. 92. Cf. President’s Address to Geological Society, 1880, ‘On the Structure 
and Origin of Non-Calcareous Stratified Rocks,’ Quart, Journ. Geol. Soe,, vol. XXxvi. 
p. 72. 
* Band ii. s, 49-58 (1878). 
