GEOLOGY beneath the crust. In those subterranean regions they have not only had to support a pile of superincumbent strata, but have been sub- jected to a vast amount of mechanical deformation by the operation of powerful earth stresses. Under the influence of pressure the incoherent particles which enter into the composition of the sediments have been compacted into solid rock. On these rocks the earth movements have produced structural modifications which have not only affected the indi- vidual beds, but also the material of which they are composed. In response to lateral pressure the strata have been bent and doubled back one over another and thrown into a set of folds closely packed together; and finally have obtained relief from the strains by actual disruption. Of the mechanical changes which these rocks have undergone, the most conspicuous is undoubtedly the production of cleavage planes, which by their action on the indurated muds have evolved the slates, the quarrying of which has formed a staple Cornish industry for at least three hundred years. Carew in 1602 described the blue roofing slates of Cornwall as ' in substance thinne, in colour faire, in waight light, in lasting strong, and generally carrieth so good regard, as (besides the supply for home provision) great store is yeerely conveyed by shipping both to other parts of the realme, and also beyond the seas, into Britaine and Netherland.' The compression to which the strata have been sub- jected has resulted in the production of planes of fissility along directions at right angles to the direction of pressure. These cleavage planes, have been among the last mechanical changes effected by the lateral strains in response to the earth stresses ; they are usually of regular character, and form a close array of parallel planes which traverse the strata, independent of the original stratification and to some extent of the folding in which that stratification has been involved. Although bearing no relation to the original bedding, their disposition in regard to the flexures is more defined, as the cleavage planes show a marked tendency to lie parallel with the axes of the folds. When the material which has been cleaved was originally a pure clay of uniform consistency, the result has been a perfect roofing slate in which the earlier structures of stratification and folding have been either completely obliterated or preserved as bands, only to be distinguished by slight variations in colour. Instead however of this homogeneous character so favourable for the formation of slate adapted for economic purposes, these Palaeozoic deposits are mostly of heterogeneous composition, the cleavage of which exhibits marked variations, so that the slate that can be wrought as an article of commerce is restricted to limited areas. The ' killas ' of Cornwall being largely made up of alternating bands of diverse texture, its compression in lieu of effecting a uniform cleavage has produced a complex set of structures, the elucidation of which requires much patient and minute investigation. Where cleavage for instance has been set up in alternating layers strongly contrasted, such as shale and sandstone, the common type in fact which the coasts of Falmouth Bay and Gerrans Bay exhibit, the cleavage planes 1 17 3
