Page:VCH Cornwall 1.djvu/66

 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 18