384 



REPORT — 1856. 



Sedgwick seems to refer to such a case as one of imperfect cleavage, 

 marked by parallel planes at definite distances, which it might be difficult to 

 class with joints or cleavages. 



(b.) In other examples all the strata are traversed by cleavage, but not all 

 at the same angles of inclination. 



In this Diagram, taken by the author 

 from Leek Beck near Kirkby Lonsdale, 

 1823*, the letters indicate, as before, soft 

 and hard beds : the inclination of the 

 cleavage planes varies in these beds in 

 such a way, that in the harder bed they 

 deviate more from planes of stratification 

 than in the softer beds. 



Such cases were observed by the au- 

 thor in Wales, 1836; North Devon, 1839; 

 Cove of Cork, 184'3; by Sharpe in Lang- 

 dale, J849; by Townsend at Cork, 1854; 

 by Harkness in the S.W. of Ireland, 1855. 



(c.) Not unfrequently, when beds al- 

 ternate whose mineral aggregation is not 

 uniform, the cleavage surfaces are curved 

 in the remarkable manner shown in Dia- 

 gram 23. 



In this Diagram (23) the cleavage edges 

 seen in the principal section are bent, so 

 that at the surfaces of each bed they 

 tend to coincide with the stratification, 

 but in the middle of each bed they form a 

 considerable angle with the stratification. 

 The first example I ever saw of this was 

 at Sallenche, in the Liassic slate, at the 

 base of Mont Blanc, in 1830. I afterward 

 ob.'erved it at Dolbadarn, in North 

 Wales, in Westmoreland, and Devonshire. 

 Mr. Sharpe has since confirmed this state- 

 ment. It is sometimes possible to trace 

 near the bounding surfaces ol the beds 

 laminae (/) of deposition, and sometimes 

 the original distinction of beds is only 

 marked by such laminae. 



(d.) Cleavage surfaces are usually dis- 

 turbed when traversing or passing near to 

 masses of unequal hardness. 



When, as in Diagram 24', beds of slate 

 enclose nodules of greater hardness, — as 

 limestone or ' calliard,' or ironstone, — the 

 cleavage, which is perfect and continuous 

 in the mass of slate, becomes irregular and 

 interrupted so as to resemble a series of 

 cracks in the nodules ; these cracks follow 

 the law indicated in Diagram 21, and tra- 

 verse the nodules in directions more nearly 



* Geol. Trans. 182a 



