■^H 



REPORT 1856. 



Fig. 5. 

 On the River Towev. 



Cleavage dips N.W. by noith, and is parallel across many flexures. 



In Diagram 6, a case of local exception to the rule is given by Sedgwick. 

 There the cleavage planes preserve their strike, but change the direction 

 and amount of their inclination, in such a way as to pass vertically through 

 the anticlinal axis, and to be inclined toward this akis on each side of it. 

 There is no clearaye observable in the lower or more central parts of the 

 bent mass of rocks. 



Fig. 6. 

 Craig Gibbon. 



On road from Llangollen to Ruthin. Cleavage convergent to an anticlinal dipping N.N.E. 

 on one side, and S.S.W. on the other, but vertical in the axis of the strata. 



§ 4. Cleavage symmetrically related to axes of movement of (he Strata. 



In a great number of examples in Wales, Westmoreland and Yorkshire, 

 where the cleavage is perfect and tiie strata are distinct, it is found that the 

 edges of the laminae of cleavage show themselves very plainly in the surfaces 

 of stratification, and these edges are often nearly horizontal. To use the 

 expression of Sedgwick, who first declared the fact, " where the cleavage is 

 well developed in a thick mass of slate rock, the strike of the cleavage is 

 nearly coincident with the strike of the beds*." This is most frequently ob- 

 served where the strike of the strata is most persistent ; or in other words, 

 where the anticlinal and synclinal axes of movement are most simple, con- 

 tinuous, and uniform in direction. 



But where the axes of movement are complicated by small folds and 

 twists, the local coincidence of the strike of cleavage and the strike of stra- 

 tification frequently fails ; the cleavage maintains, or tends to maintain, one 

 uniform direction, and tiius crosses the folds of the strata under various 

 circumstances, more or less suggestive of an influence more general than 

 that which determined the folds. 



If the expression above quoted from Prof. Sedgwick be well considered, 

 and taken in connexion with the exceptions which he mentions, it will appear 

 that in his mind the direction of cleavage in a large district was coincident, 



* Geol. Trans. 2nd series, vol. vi. p. 473. The \v«rd "strike" was, I believe, first era- 

 ployed in this sense by Sedgwick. 



