384 



planes, contribute not a little, the author conceives, to the illusive 

 appearance of a general inward dip of all the strata, even the newest, 

 under the older formations of the high igneous crests of the chain ; 

 for both the cleavage planes and the crystalline foliation observe a 

 very constant parallelism in the direction of their dip to the dip of 

 the axis planes of the flexures. 



Slati/ ^Cleavage. 



It is now a good many years since Professor Sedgwick and other 

 geologists announced the important general fact, that the structure 

 called slaty cleavage pervades the altered strata affected by it in di- 

 rections independent of their bedding or laminae of deposition ; that 

 these planes of cleavage are approximately parallel to each other 

 over large spaces of country, however contorted the dip of the rocks ; 

 and that where the cleavage is well developed in a thick mass of 

 slate rock, the strike of this cleavage is nearly coincident with the 

 strike of the beds. Professor Phillips, in 1843, added to this rule 

 a still more comprehensive and exact expression — that the cleavage 

 planes of the slate rocks of North Wales were always parallel to the 

 main direction of the great anticlinal axes. Since 1837j these phe- 

 nomena of the close parallelism of the cleavage planes with each 

 other, and with the main axes of elevation, have been observed and 

 recorded by Professor W. B. Rogers and the author of this commu- 

 nication ; and in 1849 the author submitted to the American Asso- 

 ciation for the Advancement of Science a communication on the 

 analogy of the ribbon structure of glaciers to the slaty cleavage of 

 rocks, in which he stated what he deems the true law of cleav- 

 age of a district of undulated and plicated strata, — namely, that 

 the cleavage dip is parallel to the average dip of the anticlinal 

 and synclinal axis planes, or those planes which bisect the flexures. 

 The generality of this rule was shown by sections exhibiting the 

 flexures and cleavage in the Appalachians, in the Alps, and in the 

 Rhenish Provinces. Subsequent observations in other localities 

 have confirmed the universality of this law , and the recent descrip- 

 tion of the Devonian strata in the south-west of Ireland by Profes- 

 sors Harkness and Blyth still farther tend to illustrate and establish 

 it. In their paper in the Edinburgh New Philosophical Journal 

 (October 1856), they not only recognise an agreement between the 



