1846.] SHARPE ON SLATY CLEAVAGE. 99 



when the heated mass cooled and a contraction took place in con- 

 sequence. 



The undulating surface of the cleavage over the central parts of 

 Devonshire in low waves which either coincide with more boldly 

 arched waves of the bedding, the two having a common axis parallel 

 to the boundaries of the area, or which cut obliquely through the 

 curves of the bedding, is a phsenomenon which requires for its 

 explanation the depression of the centre of the area after the forma- 

 tion of the cleavage. The elevation of the bedding in folds parallel 

 to the axis might be regarded as part of the original movement; but 

 not so the undulations of the cleavage ; and the peculiarity, that the 

 waves of the bedding and cleavage sometimes coincide in direction 

 and sometimes differ, refers them to two operations, of which the 

 cleavage being the less disturbed seems only to have undergone the 

 later one. This undulation of the cleavage has only been noticed 

 over the centre of the flat arch of elevation of Devonshire : in di- 

 stricts where the beds and the cleavage planes are tilted at considera- 

 able angles, a lowering of the arch might have taken place which 

 it would be difficult to detect. 



The foreign crystalline matter which is frequently found between 

 the planes of cleavage, shows that at some period after the cleavage 

 was completed the rock had a tendency to gape along them. Mr. 

 Darwin has collected many instances of the kind from his own ob- 

 servations and those of Professor Sedgwick*, of which the most re- 

 markable is the occurrence of trap dikes in Tierra del Fuego between 

 the laminae of the slates f. 



In the cliffs at the foot of Hillsborough and in other places near 

 Ilfracombe, many instances may be seen of the insertion of sheets of 

 quartz between the laminae of slate, as shown in the section fig. 21 . 



Fig. 21. 



XXKSX 



The horizontal lines represent the beds, the oblique dotted lines the cleavage, and the thick black 

 lines the sheets of quartz. 



The quartz, which is often an inch thick, lies partly between the beds 



* Darwin, Geological Observations on South America, pp. 152, 160 and 163 

 Sedgwick, Transactions of the Geological Society, 2nd Series, vol. iii. p. 471. 



+ Had I been aware of this observation, 1 should have sought for similar phae- 

 nomena among the greenstone dikes of Carnarvonshire, which strike with the 

 cleavage and bedding of the district, but of which the dip has not been satisfactorily 

 ascertained. 



H 2 



