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III.—On Cleavage and Distortion

Published online by Cambridge University Press:  01 May 2009

O. Fisher
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Extract

The history of opinion upon the subject of cleavage seems to be this. Sedgwick had come to the conclusion that cleavage was a condition, “impressed on certain rocks and in certain regions by the operation of some very extensive cause, after the stratified rocks had undergone great displacement,” and he suggested that the cause might be some kind of crystallization. But Dr. Sorby discovered that the intimate structure to which cleavage is due is not crystallization, but a mechanical rearrangement of the particles of the rock: and he produced it artificially by squeezing clay containing flaky particles. Dr. Tyndall did the same thing with different substances, without introducing flaky particles. In either case it was by pressure that the result was obtained. Seeing therefore that pressure could succeed in laboratory experiments, and that cleavage in the field was ordinarily nearly at right angles to the direction in which the action of pressure might have folded the strata, it was natural to conclude that both folds and cleavage had been concomitant effects of such a pressure. And this appears to be the theory now generally adopted; and is of course in opposition to both of Sedgwick's conclusions. For my part I believe that Sedgwick was right in thinking that cleavage was produced by some very extensive cause operating after the rocks had undergone great displacement; while I agree with Sorby and Tyndall in attributing it to a mechanical action, accompanied with pressure. Nevertheless I do not think that this pressure was the same exhibition of force, that originally folded and elevated the rocks, as explained in Part IV. Thus I am in accord with each of these authors in the domain in which his authority stands highest.

Type
Original Articles
Information
Geological Magazine , Volume 1 , Issue 9 , September 1884 , pp. 396 - 406
Copyright
Copyright © Cambridge University Press 1884

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References

page 397 note 1 Phillips' Report on Cleavage, 1856, Brit. Assoc. Reports, 1857, p. 373.

page 397 note 2 Geological Magazine, 1884, p. 271.

page 397 note 3 Since sending my MS. to the Editor, I have enquired of Mr. Pengelly, “What is the relation of the cleavage to the unconformity ?” His reply is as follows:—

“The cleavage in the section at Hope's Nose affects both sets of rocks, and it is not diverted by the uncunformity. The following is briefly the reading of the section:—

1st. The formation of the lower series by the deposition of triturated organic exuviae; the process being frequently intermitted, as is proved by the interstratification of volcanic ash.

2nd. By the operation of some approximately lateral force, the beds were contorted, and were fractured at the point of greatest flexure.

3rd. The uppermost beds of this lower series were exposed to the action of the waves, and planed down to an approximately horizontal surface.

4th. After this denudation, the beds of the upper series were deposited unconformably on the lower, and are now nearly horizontal.

5th. Though these processes must have absorbed a large amount of time, they were all completed within one and the same division of the Devonian period, as is proved by the specific identity of the numerous fossils iu the two series.

6th. After the deposition of the upper series, cleavage was set up in the entire mass.

7th. Assuming that cleavage is due to pressure, there are conditions under which pressure can produce contortion without cleavage, and conditions under which it can produce cleavage without contortion.”

page 398 note 1 On certain Movements in the Parts of Stratified Rocks, Brit. Assoc. 1843. Quoted by Sharpe, loc. cit. p. 76.

page 398 note 2 Phil. Mag. vol. xii. p. 198.Google Scholar

page 399 note 1 Quart. Journ. Geol. Soc. vol. iii. 1846.Google Scholar

page 400 note 1 Phil. Mag. Jan. 1856.

page 400 note 2 The argument (p. 273) regarding the circumstances which would lead to a viscous shearing as distinguished from faulting supposes that a horizontal stress causes a shearing of the parallelepiped. We now regard the shearing as causing a stress. The relations of the forces will remain unaltered, although the directions of the motions will be reversed.

page 400 note 3 Edin. New Phil. Journ. 1853, vol. iv. p. 138, copied by Professors Tyndall, and Phillips, .Google Scholar

page 402 note 1 Equation 3.

page 404 note 1 “In just such a manner as would occur if the dimensions of the slate had been changed as previously mentioned.”—“On the Origin of Slaty Cleavage,” Edin. New Phil. Journ. vol. lv. p. 140, 1853.Google Scholar

page 404 note 2 Text-Book of Geology, p. 310.

page 405 note 1 Manual of Geology, p. 271, ed. 1862.