Ch. XXXVL] slaty cleavage. 603 



to one position, that must of course determine a cleavage-plane. Thus 

 we see the infinitesimal crystals of fresh precipitated sulphate of barytes, 

 and some other such bodies, arrange themselves alike in the fluid in which 

 they float ; so as, when stirred, all to glance with one light, and give the 

 appearance of silky filaments. Some soils of soap, in which insoluble 

 margarates* exist, exhibit the same phenomenon when mixed with 

 water ; and what occurs in our experiments on a minute scale may occur 

 in nature on a great one." f 



Professor Phillips has remarked, that in some slaty rocks the form of 

 the outline of fossil shells and trilobites has been much changed by dis- 

 tortion, which has taken place in a longitudinal, transverse, or oblique 

 direction. This change, he adds, seems to be the result of a " creeping 

 movement" of the particles of the rock along the planes of cleavage, its 

 direction being always uniform over the same tract of country, and its 

 amount in space being sometimes measurable, and being as much as a 

 quarter or even half an inch. The hard shells are not affected, but only 

 those which are thin.J Mr. D. Sharpe, following up the same line of in- 

 quiry, came to the conclusion, that the present distorted forms of the 

 shells in certain British slate rocks may be accounted for, by supposing 

 that the rocks in which they are imbedded have undergone compression 

 in a direction perpendicular to the planes of cleavage, and a correspond- 

 ing expansion in the direction of the dip of the cleavage. § 



More recently (July, 1853), Mr. Sorby has demonstrated the great 

 extent to which this mechanical theory is applicable to the slate rocks 

 of North Wales and Devonshire,! districts where the amount of change 

 in dimensions can be tested and measured by comparing the different 

 effects exerted by lateral pressure on alternating beds of finer and coarser 

 materials. Thus, for example, in the accompanying figure (fig. 7 08), it 

 will be seen that the sandy bed e?/, which has offered greater resistance, 

 has been sharply contorted, while the fine-grained strata, a, b, c, have 

 remained comparatively unbent. The points d and/ in the stratum d f 

 must have been originally four times as far apart as they are now. They 

 have been forced so much nearer to each other, partly by bending, and 

 partly by becoming elongated in the direction of what may be called 

 the longer axes of their contortions ; and lastly, to a certain small amount, 

 by condensation. The chief result has obviously been due to the bend- 

 ing ; but, in proof of elongation, it will be observed that the thickness 

 of the bed df is now about four times greater in those parts lying in the 

 main direction of the flexures than in a plane perpendicular to them : 



* Margaric acid is an oleaginous acid, formed from different animal and vege- 

 table fatty substances. A margarate is a compound of this acid with soda, 

 potash, or some other base, and is so named from its pearly lustre. 



f Letter to the author, dated Cape of Good Hope, Feb. 20, 1836. 



% Report, Brit. Assoc, Cork, 1843, Sect. p. 60. 



§ Quart. GeoL Journ. voL iii. p. 87. 1847. 



\ On the Origin of Slaty Cleavage, by H. C. Sorby, Edinb. New PhiL Journ. 

 1853, vol. lv. p. 137. 



