Page:VCH Cornwall 1.djvu/67

 GEOLOGY layers which form the beds have been thrown into a set of minute folds, the arches of which have been broken by tiny cleavage planes which resolve themselves into miniature faults, and there is the same tendency to override that we see in the larger divisions of the beds. On examination the rock is frequently seen to be full of these little folds and thrust planes, with a disposition to a secondary cleavage, while minor movement planes appear in the more resisting core itself with accompanying strain-slip cleavage ; and the cleavage planes which pass through the axes of the minute folds often culminate in small thrusts. But besides the structures we have enumerated, these crushing processes acting on heterogeneous strata have produced in Cornwall a set of widely distributed breccias which closely simulate the coarser products of erosion. These breccias or pseudo-conglomerates are well developed on the western side of the Carrick Roads, in a belt that extends from Feock to Falmouth and is broken by the estuaries which form the creeks of Penryn, Mylor and Restronguet, along the shores of which they may be conveniently studied. They consist of slate fragments enclosed within the strata ranging from the size of peas up to 5 or 6 inches in length, with their flat sides lying in more or less parallel planes. As a rule the larger fragments are angular, while the smaller are sub-angular and may sometimes be per- fectly rounded. They may consist of either argillaceous or siliceous material, and are identical in composition with the matrix of the slate in which they are enveloped, and from which they have been obviously derived. Instead however of being water-worn as their appearance suggests they owe their origin to agencies very much more complex, and represent a phase of those processes of deformation to which we have drawn attention, so that instead of being an original structure of the rock, they present a most striking record of its subsequent deformation. The Mylor beds, in which these phenomena are best displayed, are made up of dark blue argillaceous and fine quartzose beds which succeed one another in such thin alternations that the strata are conspicuously striped. The changing nature of these interlamina- tions, and the corresponding variation in their limits of compression, have resulted in different degrees of resistance to the crustal movements ; the softer beds having easily yielded, while the more resistant strata] separated from each other by bands which are beginning to yield, and being thus deprived of support, are smashed, and the fragments become involved in the more yielding mass. Under the influence of these movements the particles not only become detached from the parent rock, but are frequently rolled in the process and simulate pebbles. If we endeavour to trace the normal laminated beds into the fragmental or brecciated type, we see that the former gradually lose their regular appearance, and become affected by small folds and thrusts, until at last they are nothing more than a mass of segments more or less detached, and it becomes 19