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CHAPTER XIX

SLATY-CLEAVAGE

THE experiments which have been detailed in

the previous chapter prove that under compressive stresses the beds or strata-platesminiature representations of portions of the earth's lithosphere-undergo complex movements, and are bent, twisted, thickened, and even stretched in the process. In adapting themselves to these new conditions the materials of which the beds are composed flow and shear in directions governed by the thickness or thinness of the plates, the direction and strength of the compressing forces, and the plasticity or rigidity of the materials. In Nature there is a vastly greater variety of material in assembled series than are represented in our models. There may be fine clay, granular sedimentary deposits, conglomerates, volcanic beds, igneous dykes and sills, or the wonderful combination of compressed granitic, felsitic, and other intrusive igneous rocks usually constituting an Archæan complex.

When the compressing forces which affect the earth's crust from time to time act upon these heterogeneous beds, the variety of movement and

resulting forms must of necessity be greater than in our clay models.

Instances of parallelisms of bedding planes over large areas that have been subject to great compression can, however, be pointed to. The South of Ireland is an example, which will presently be dealt with.

Parallel Structure.-The movements illustrated in the clay models subjected to compression are, mainly, folding and shearing of a more or less complicated and often convoluted type, and it has been shown that this involves flow of material from one place to another in diverse rather than parallel directions.

The structure known to geologists as slatycleavage is eminently a parallel structure, which cuts through folds, convolutions, and even twists in strata with such remarkable indifference that the normal bedding or planes on which the rock was originally laid down become practically obliterated as a structure. The rock no longer splits along its bedding planes, but along the induced parallel structure, which ignores the original conditions of deposition. There is, however, one great fact, often lost sight of, namely, that all classes of rocks cannot develop slaty-cleavage, however much they may be compressed or subjected to identical influences.

It is here that the mechanical theory, which attributes slaty-cleavage to pressure, and pressure alone, fails.

Why this should be will form one of the aims of our inquiry.

INVESTIGATIONS OF SLATY-CLEAVAGE

The subject of slaty-cleavage had long possessed great attractions for me, and after the publication of 'The Origin of Mountain Ranges' I further studied it in the field and collected typical specimens.

It was not, however, till 1897 that the investigations about to be described were undertaken in collaboration with Mr. Philip Holland, F.I.C.

The first result of our work was the publication of a paper on the 'Phyllades of the Ardennes compared with the Slates of North Wales,' Part I.,1 followed by Part II. in 1900,2 and in 1901 by 'The Green Slates of the Lake District.' In the two latter we were kindly assisted in the microscopical work by Mr. Maynard Hutchings, to whom we wish to express our indebtedness.

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In the last of these papers we sketched out a Theory of Slate Structure and Slaty-cleavage,' founded upon our investigations, and as it in substance contains our present views I have thought it simpler and better to reproduce it verbatim than to recast it specially for this work.

Since the publication of this theory we have carried on further investigations, the substance of which will be found recorded in the ensuing pages.

For the sake of completeness, and to add to the

1 Proceedings of Liverpool Geological Society, 1898, pp. 274–93. 2 Ibid., 1900, pp. 463–78. 3 Ibid., 1901, pp. 101-27.

practical value of the work, I reproduce at the end of this chapter the whole of the tables of chemical analysis published in the papers to which I have already referred.

A THEORY OF SLATE STRUCTURE AND SLATY-
CLEAVAGE BY READE AND HOLLAND 1

The Chemical and Mineralogical Make-up of Slates. It will be noticed on careful examination that, in all the examples of slates analysed by us and reported upon mineralogically by Mr. Hutchings, slaty-cleavage and the development of micaceous and chloritic minerals appear to be indissolubly associated. These minerals have been 'rolled out' in the direction of the cleavage planes. They are evidently secondary, and we cannot but think there is good evidence to show that they have been developed pari passu with the movement of the constituent particles of the rock which has given way under shearing stresses. High temperature, pressure, and movement acting upon a rock-base chemically and mechanically fitted for the manufacture of the slate, if we may be permitted to use that expression, seem to be a necessary conjunction for the production of slatycleavage, and the more foliaceous the constituent particles, and the more abundantly they are present, the firmer and more perfect will be the cleavage.

1 From Proceedings of Liverpool Geological Society. 1901, pp. 120-27.

The rougher slates are made up of larger clastic or crystal grains or particles, and the whole aggregate, whether large or small, is made intimately coherent by the presence of secondary minerals of a foliaceous character, which act not only as structural parts of the slate and longitudinal ties, but also as a cement.

We have already pointed out that, in our view, the slaty-cleavage has been impressed upon the rock in a late stage of its individual history. In confirmation of this we may point to the extraordinary distortions the banded rocks of Tilberthwaite and Buttermere have undergone 2-a distortion that, had it occurred after the cleavage structure was developed, would have affected the cleavage planes themselves.

This has not occurred, and it is this great fact of the cleavage planes in all slate rocks traversing the rock in parallel planes, quite independently of the bedding, that inclined the great Sedgwick, who was the first to study the phenomena scientifically, to think that cleavage was a sort of crystallisation on a large scale, or due, as he expressed it, to crystalline or polar forces acting on the whole mass simultaneously in given directions and with adequate power. 13

Old and New Views and Theories of Slatycleavage tested and compared.-This short sum

1 Part II., Phyllades,' &c., p. 476.

2 Pages 107 and 110.

3 On the Structure of Large Mineral Masses' (Trans. Geo. Soc. vol. iii., 1831).

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