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 



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