ON CLEAVAGE AND FOLIATION IN BOCKS. 375 



or nearly so, with the main or mean direction of the strike of the beds, though 

 it is not actually so stated in the paper. In 1843 I presented as the result 

 of a special study of the geographical relation in question, among the 

 slaty rocks of Wales, the following explicit expression, — " The cleavage 

 planes of the slate rocks of Wales are always parallel to the main direction 

 of the great anticlinal axes, but are not affected by the small undulations 

 and contortions of those lines*" which may be regarded as confirming the 

 views of Sedgwick. Prof. Jukes finds the same result in Newfoundlandf . 

 Mr. Darwin has an analogous expression for South America : — " The clea- 

 vage laminae range over wide areas with remarkable uniformity, being parallel 

 in strike to the main axes of elevation, and generally to the outlines of the 

 coast |." And since 1837, Professors H. D. Rogers and W. B. Rogers have 

 observed and recorded, in Virginia, Pennsylvania, and New Jersey, " the close 

 parallelism of the cleavage planes of a given district with each other, and 

 with the main axis of elevation of the district §." And lastly, in 1849, Mr. D. 

 Sharpe, in reviewing these statements, adds, as from his own conviction, that 

 " the direction of the strike of the cleavage is parallel to the main direction 

 of the axes of elevation, and has no necessary connexion with the strike of 

 the beds||." This is somewhat enigmatical, for it is by the "strike of the 

 beds" that we determined the axes of elevation and depression : Mr. Sharpe 

 had perhaps misunderstood Professor Sedgwick's use of the word strike, and 

 probably meant to say that the cleavage observed at any one place was not 

 necessarily dependent on the strike of the beds at that place. Professor 

 Harkness has found remarkable agreements between the strike of cleavage 

 and the axes of movements in the S.W. of Ireland ^. According to these 

 authors, then, though cleavage is really or nearly independent at every 

 point of the previously fixed position of the strata there, crosses them with 

 little variation, whether they be curved or plane, and preserves or nearly 

 preserves its own dip or its own verticality, in whatever direction and in 

 whatever degree they are inclined ; cleavage and stratification have, never- 

 theless, one real geographical relation, an approximate parallelism of strike, 

 dependent on the axes of movement of the rocks. To this conclusion, how- 

 ever, there are many exceptions ; one of the most remarkable exceptions 

 known to mu is found in North Devon, where the general strike of the beds 

 is nearly east and west ; but the cleavage strike is nearly E.N.E. and W.S.W., 

 by the observations of Sedgwick, Sharpe and myself. 



In Charnwood Forest I find the average strike of the strata, exactly 

 measured, to form an angle of 19° 12' with the average strike of the cleavage. 



§ 5. Relation of Cleavage Planes to the Inclination of the Strata. 



Almost every observer in mountainous regions who has once perceived the 

 symmetrical relation of the strike of cleavage to the great axes of movement 

 of the masses, seeks for some corresponding symmetry between the dip of 

 the strata and the inclination of the cleavage. But unless the investigation 

 be carried across a whole district, so as to furnish comparisons on both 

 sides of all the anticlinals and synclinals, the result cannot be much relied 

 on. Mr. Darwin, who has in this respect the advantage of great range of 

 observation, having observed the persistence of the strike of cleavage, and 



* Reports of the British Association, 1843, p. 61. 



t Geological Survey of Newfoundland, p. 130. 



% Geological Observations in South America, p. 162. 



§ Ann. Reports on the Surveys of these States, 1837-40. 



II Proceedings of Geol. Soc. 1846. 



^ Reports of British Association, 1855, p. 82. 



