92 REPORT—1857. 
On some Facts connected with Slaty Cleavage. 
By H. C. Sorsy, F.BS., F.GS. 
This communication contained an account of some of the general conclusions the 
author had arrived at since the publication of his papers on the subject. The micro- 
scopical mineral structure of much so-called clay-slates, is entirely different from the 
modern deposits of clay, formed from decomposed felspar, or from those occurring in 
strata that have undergone no subsequent chemical change. In fact they contain 
little or no clay, according to the usual acceptation of the term—no felspar clay,—but 
are often almost entirely composed of very minute plates and crystals of a peculiar 
mica, so that they might be called mica clay-slates, Though differing materially in 
several particulars, they are thus analogous to very fine-grained mica-schists, into 
which they gradually pass by the increase in the size of the crystals of mica. The 
form of many of the particles of which they are composed, and the manner in which 
they are arranged, are quite different from what are seen in rocks that have been 
merely deposited from suspension as mud, but are extremely like what occur when 
minute crystals are formed in sifu. When not altered by subsequent mechanical 
movements, the structure most closely resembles that of pseudomorphs of mica or 
chlorite after felspar. If theu it be supposed that the material had been at first depo- 
sited as a decomposed felspar clay, and that a subsequent alteration occurred, so as to 
convert it into a mass of minute crystals of mica, the physical structure could easily 
be explained ; and it would not be requisite to suppose that the deposit was originally 
in any way different from the ordinary clays of more modern periods. That this 
change is physically possible is proved by the occurrence of large crystals of felspar, 
entirely replaced by mica and quartz; and that the chemical composition of a deposit 
of partially decomposed felspar is the same as that of a mixture of mica and quartz, 
may be readily seen by comparing their composition. — This may very clearly be shown 
by employing the very simple formulz for felspar, mica, and clay adopted by Gmelin 
in his ‘ Handbook of Chemistry’ (Cavendish Soc, translation, vol. iii. pp. 415, 441 
and 449). 
2 equivalents of clay = 48:1=2Al,0,+ 45i0,+4aq 
md “A of felspar = 51°9= Al,O;+ 6Si0,.4+KO 
100-0 3A], 0,4+10Si0,+KO+4aq 
f 
1 equivalent of mica = 70°8=3Al, O,+ 6Si0,+KO 
4 equivalents of quartz = 22°6= 4Si0, 
4 equivalents of water = 66= 4aq 
100°0 3Al, 034+10Si0,+ KO+4aq 
The presence of the oxides of iron and other bases in the clay would, of course, mate- 
rially modify these results ; and when sufficient magnesia was present, it probably 
determined the formation of chloritic or steatitic slate. 
The author therefore thinks that such clay-slate as is almost entirely composed of 
mica was originally a deposit of ordinary felspar clay, and that, probably under the 
action of water at a high temperature, this was altered into a mass of minute crystals of 
mica. ‘The quartz appears to have either remained disseminated amongst the mica, 
or to have been removed and deposited in other situations ; for quartz veins are of very 
eneral occurrence where this change has occurred, but are usually absent from those 
strata where the felspar-clay remains in its original condition. ‘This view of the sub- 
ject is of much interest in connexion with slaty cleavage, since it explains why the 
minute particles of mica were arranged so very promiscuously in all directions before 
the cleavage was developed by pressure; a fact which presented great difficulty, when 
it was supposed that they had been simply deposited as mud from suspension in 
ter. 
we The author then proceeded to point out that there are two distinct extreme kinds 
of structure that have often been confounded under the term slaty cleavage. One of y: 
these, characteristic of the best roofing-slates of Wales, is an ultimate structural 
weakness, quite independent of any actual fractures or breaks of continuity, and may 
thus be called wltimate-structure-cleavage. Experiments clearly prove that this is 
just such a structure as would result from the rock yielding to pressure as a plastic 
12.5 v 
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