ON CLEAVAGE AND FOLIATION IN ROCKS. 373 



tracts the enormous and repeated undulations of the strata, — he found these 

 seemingly irregular structures crossed and cut through by a series of 

 planes characterized by almost unvarying symmetry — parallel and con- 

 tinuons through the heart of Snowdonia and the steeps of the Westmore- 

 land Alps, — and so regular as to appear like the results of enormous 

 crystallization. 



These results — confirmed by universal research among the mountainous 

 tracts of the old and new world — by Studer and Forbes in the Alps, by 

 Murchison in Siluria, Darwin in the Andes, and Rogers in the Appalachians*, 

 — leave no doubt that cleavage is a peculiar structure impressed on certain 

 rocks and in certain regions, by the operation of some very extensive cause 

 operating after the stratified rocks had undergone great displacement. 

 For this fundamental generalization we are, I bflieve, entirely indebted to 

 Sedgwick. 



§ 3. Cleavage in continuous parallel planes across bent and contorted 

 Strata. 



Of this remarkable fact, and of its extensive bearing on the theory of 

 cleavage. Professor Sedgwick's memoir gives the earliest notice, confirmed 

 by abundant examples in Wales : — " A rugged country, more than thirty 

 miles in length and eight or ten in breadth, stretching from the gorge of the 

 Wye above Rhaiadr to the upper gorges of the Elan and the Towy, exhibits 

 on a magnificentscale, thousands of examples of much contorted strata, crossed 

 by parallel cleavage planes. Of the true bedding in these cases there is not 

 a shadow of a doubt. Many parts are of a coarse mechanical texture ; but 

 subordinate to these are fine chloritic slate. But the coarser beds and the 

 finer, the twisted and the straight, have all been subjected to one change. 

 TtHiatever be the contortions of the rocks, 

 the planes of cleavage pass on, generally 

 without deviation, running, in parallel 

 lines from one end to the other, and in- 

 clining at a great angle to a point only a 

 few degrees west of magnetic north\." 

 The Diagram No. 3 shows the directions 

 here assigned. Those which follow (4, 5) 

 are vertical sections copied from Sedg- 

 wick, to show the parallelism of cleavage 

 planes across strata bent anticlinally (4) 

 and contorted (5). 



Fig. 3. 



Fig. 4. 

 River Wye above Rhaiadr. 



Cleavage dips to N.W., across anticlinal. 



* Proceedings of American Naturalists and Geologists, 1 845. 

 t Geol. Trans. 2nd series, vol. ii. p. 477. 



