DEFORM. 4 77 ON OF NOCKS 599 



The assumption that bedding and secondary structures cor- 

 res]jond is still less justifiable when no remaining evidence of 

 bedding is found. If only cleavage or fissility be found and the 

 relations of the beds with other beds are not such as to give the 

 direction of stratification, no inference in reference to this point 

 should be drawn. 



Fig. 12. — Closely plicated shale underlain b}' bed of limestone. 



It is apparent that attempts to estimate the real thickness of 

 cleaved or fissile beds must take into account two difficulties : 

 (i) The same bed may be folded on itself many times, and 

 these folds must be followed, or at least some estimate must be 

 made of the thickness of the beds which would be present if the 

 minute plications could be straightened. (2) In the complex 

 folding of the beds there is readjustment, mashing and conse- 

 quent lengthening of the layers upon the limbs, and they are, 

 therefore, on the average, thinner than originally. So far as 

 such thinning occurs, it compensates for the reduplication of the 

 beds, but it is believed that this compensation is far short of full 

 correction. To fully overcome these difficulties is often impossi- 

 ble, and estimates of the thickness of closely folded, cleaved, 

 and fissile beds, even when. all the difficulties are wholly under- 

 stood and allowances made, are usually only approximate. 



DEVELOPMENT OF CLEAVAGE BY OTHER CAUSES THAN THRUST. 



Thus far I have considered cleavage developed in connection 

 with and dependent upon orogenic movements. It is probable 

 that this structure develops in other wa3's. It ma}^ be that 

 deeply buried beds ma}- become cleavable with the structure 

 parallel to bedding, where superincumbent pressure, cementa- 

 tion, and metasomatic changes are the predominant forces. 



