376 



REPORT 1856. 



the frequent change of their dip both in angular value and direction, sought 

 for some order in these changes. He observed that frequently, in Tierra 

 del Fuego and in other countries in South America, cleavage planes were 

 inclined in opposite directions on opposite sides of an anticlinal, so as to dip 

 inwards*. The Alps, given as an example of this fan-like arrangement of 

 strata by Studerf , and the corresponding appearance previously recorded 

 by von Buch in Norway, are mentioned by Darwin as possibly related to 

 this phaenomenon of cleavage. Professor H. Rogers submitted to the Ame- 

 rican Association for the Advancement of Science, a further statement, that 

 "the cleavage dip is parallel to the average dip of Hie anticlinal and synclinal 

 axis planes, or those bisecting the flexures J." The Alps in this view are 

 supposed to have on their flanks many folds of strata, whose " axis planes" 

 dip inwards; and parallel to these "axis planes "the cleavage structure is 

 developed. The axis planes are more highly inclined at greater distances 

 from the central summit ridge. 



Fig. 7. 

 Hypothetical Sketcli Section of Alps. 



For the most detailed view yet presented on this subject we are indebted 

 to Mr. D. Sharpe, now unhappily lost to science. According to Mr. Sharpe, 

 if we trace geographically any particular plane of cleavage by following its 

 strike 5, 10, 20 or more miles, we shall find it preserve, within narrow limits, 

 the same angle of dip, and in the same direction. On proceeding a few miles to 

 the right or left, and selecting a second plane of cleavage, it is probable that 

 this will not dip at the same angle, possibly not in the same direction ; but 

 this angle and this direction of dip are equally persistent along the line of 

 strike to which they belong. When by repeated trials of this kind the struc- 

 ture of a large tract of country is ascertained, it is found that along certain 

 lines of strike some miles apart, the cleavage is vertical, or nearly so ; that 

 near these lines the cleavage surfaces are steeply inclined toward them, but 

 far from thera greatly inclined. Thus something like anticlinal and synclinal 

 axes appear, and " systems of cleavage " are traced through countries which 

 also manifest " systems of movement." 



Thus Mr. Sharpe states, that in North Wales a line of vertical cleavage 

 runs N.E. and S.W. along the slate beds which lie on the western flank of 

 the Snowdon chain ; another such line runs through the great slate quarries 

 between Dinas Mowddy and Mallwyd. These lines are about 35 miles apart. 

 Between them the cleavage is inclined, — near the north-western line the dips 

 are north-westward, — near the south-eastern line they are south-eastward, — the 

 angle of inclination being least towards the middle part of the area included 



* Geological Observations in South America, p. 164. 

 t Edinb. New Phil. Journal, vol. xxxiii. p. 144. 

 J Trans. Roy. Soc. Edinb. 1856, p. 447. 



