A HISTORY OF CORNWALL 



change their character according to the nature of the layer which they 

 traverse. Those which cross the slate are not only more numerous 

 than those which traverse the sandstone, but incline at different angles ; 

 so that two shale bands which are divided by a sandstone display parallel 

 cleavage, but this parallelism is broken in the interposing sandstone. 

 Moreover if the latter is sufficiently massive it has resisted cleavage alto- 

 gether, so that we see the phenomena of cleavage in perfect parallelism as 

 regards the upper and lower members while it is absolutely severed by a 

 band in which this structure is wanting. As the sandy beds present every 

 gradation in their texture, there are corresponding gradations in the degree 

 of fissility which they exhibit. Another factor which constitutes a dis- 

 turbing element and still further varies the cleavage, is the disruption 

 of the rocks the fractures produced having acted as planes of relief 

 bringing about a cessation of the stresses in their neighbourhood so that 

 beds of similar strength present different degrees of fissility. In study- 

 ing the effects of cleavage on the Cornish strata the fact must be borne 

 constantly in mind that flexure, fracture and cleavage are intimately 

 related, and express different phases in the history of their deforma- 

 tion by the same stresses. The coast sections already alluded to dis- 

 play marked variation in the degree of deformation both as regards 

 the intensity of folding and the nature of the fissility in strata of 

 similar lithological type. 



While the cleavage has resulted merely in the flattening of the 

 component particles of the strata, as illustrated by the distortion of the 

 fossils, the rearrangement of our ' killas ' formations does not always 

 stop at this comparatively simple process, but the crushing to which 

 they have been subjected has set up interstitial movements. These 

 movements resolve themselves into a succession of small slips along 

 the cleavage planes, and have often been sufficiently severe to set up a 

 transverse cleavage on their own account ; the latter in many cases so 

 well developed as to form the dominant cleavage of the rock, and in its 

 turn to have acted as planes for further interstitial movements. 



In response to lateral pressure the strata have been folded, the folds 

 have been closely packed together, bringing their limbs into a general 

 parallelism, so that no further relief from pressure is to be obtained by 

 plication. But the strength of the rock sometimes fails before the stresses 

 which still continue to bear, and rupture occurs. The fractures still follow 

 the disposition of the folds, and snap their arches, the planes so formed 

 allowing one part of the mass to override that which adjoins it. So that 

 the folding stage has been replaced by a phase of fracture which takes the 

 form of thrusts by which individual segments are pushed forward. The 

 reversed faults so formed are known as thrusts, to distinguish them from 

 normal faults, in which the ruptured rock segments have dropped down- 

 wards. 



While the strata on the large scale have been thus modified, their 

 component particles have been undergoing on the small scale precisely 

 the same process which has set up interstitial movements. The individual 



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