750 



SLATE ROCK OF NORTH DEVON. 



[Ch. XXXVI. 



Fig. T60. 



being as much as a quarter or even half an inch. The hard shells are 

 not affected, but only those which are thin.* Mr. D. Sharpe, follow- 

 ing up the same line of inquiry, 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 im- 

 bedded have undergone compression in a direction perpendicular to 

 the planes of cleavage, and a corresponding expansion in the direction 



of the dip of the cleavage.f 



Subsequently (1853) Mr. Sorby 

 demonstrated the great extent to 

 which this mechanical theory is 

 applicable to the slate rocks of 

 North Wales and Devonshire,! dis- 

 tricts where the amount of change 

 in dimensions can be tested and 

 measured by comparing the differ- 

 ent effects exerted by lateral pres- 

 sure on alternating beds of finer and 

 coarser materials. Thus, for exam- 

 ple, in the accompanying figure (fig. 

 760) it will be seen that tne sandy 

 bed df y which has offered greater 

 resistance, has been sharply con- 

 torted, while the fine-grained strata, 

 a, b, c, have remained comparatively 

 unbent. The points d and / in the 

 stratum df must have been origi- 

 nally 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 condensa- 

 tion. The chief result has obviously 

 been due to the bending; but, in 

 proof of elongation, it will be ob- 

 served 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 ; and 

 the same bed exhibits cleavage-planes in the direction of the greatest 



(Drawn by II. 0. Sorby.) 



Vertical section of slate rock in the cliffs 



near Ilfracombe, North Devon. 



Scale one inch to one foot. 



a, &, c, e. Fine-grained slates, the stratifi- 

 cation being shown partly by lighter, or 

 darker colors, and partly by different de- 

 grees of fineness in the grain. 



d, f. A coarser-grained light-colored sandy 

 slate with less perfect cleavage. 



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

 f Quart. Geol. Journ., vol. iii. p. 87, 1847. 



% On the Origin of Slaty Cleavage, by H. C. Sorby, Edinb. New Phil. Journ, 

 1853, vol. Iv. p. 137. 



