1846.] SHARPE ON SLATY CLEAVAGE. 77 



The fossils are most abundant in Mr. Bishop's quarry. The beds 

 generally dip about 5° to the W.S.W., but they are occasionally a 

 little bent, so that the dip varies in different parts of the quarry. 

 The prevailing dip of the cleavage is to the W.N.W. at 10°; in 

 some parts the bedding and the cleavage have been slightly displaced 

 in the same direction, but this is not usual, and the angle between 

 them is not constant, varying from the smallest angle up to about 10° ; 

 in some spots the bedding and cleavage coincide in direction. These 

 differences in the angle at which the cleavage intersects the bedding 

 appear to have had an influence on the amount of distortion of the 

 organic remains, as these are usually most altered in form where the 

 angle of incidence of the two planes is the least. The most abundant 

 fossil at Tintagel is that which Mr. Sowerby has named Spirifer 

 giganteus'^ ; butthere are also some Terebratulee, stems of Encrinites, 

 &c. : all the shells are more or less distorted, but the impressions of 

 the encrinite joints are rarely altered. 



The South Petherwin limestone quarries are worked upon the 

 line of an anticlinal axis running nearly E. and W. ; but the beds 

 dip at different angles on the opposite sides of the quarries, and 

 there are also several minor faults. The planes of cleavage are also 

 bent over in a flat arch, of which the axis running E. and W. coin- 

 cides in direction with the axis of the beds, but the cleavage planes 

 are less inclined than the beds. From these irregularities the angle 

 of incidence of the planes of bedding and cleavage varies in different 

 parts, between 20° and 90°. The upper beds consist of rotten, grey, 

 argillaceous slate, separated by thin beds of soft cchreous earth, 

 crowded with organic remains too frail to bear removal, but whose 

 forms may be seen on the spot. There are many fossils in the slates, 

 especially near their junction with the ochreous beds. Below these 

 are alternations of beds of limestone and slate, containing fewer 

 fossils. The beds of different mineral character have been differ- 

 ently affected by cleavage, which is most seen in the upper slates, 

 and only affects the limestones slightly, but is hardly to be traced in 

 the thin beds of ferruginous or ochreous earth. The distortion of 

 the organic remains in each bed is in proportion to its cleavage ; in 

 the slates they are extravagantly distorted, in the limestones slightly 

 so ; while in the soft ochreous beds, in which there is little cleavage, 

 the fossils have preserved their form nearly unaltered. Although at 

 present the loss of the calcareous matter of the shell has left these 

 .thin beds softer and more rotten than the others, it is possible that 

 when they were full of shells they were better able to resist pressure 

 than the other beds, which are now the hardest. The encrinite 

 joints are usually free from distortion, although nothing remains but 

 the hollow impressions left by them : apparently when the distortion 

 took place these joints w^ere still solid bodies, capable of resisting 

 the force to which the other fossils yielded. This exception does 

 not hold good everywhere : in the slaty beds to the north of Barn- 

 staple, the impressions of the stems of encrinites are often as much 

 compressed as the other fossils. The species found at South Pether- 

 * Geol. Trans. 2nd Series, vol. v. pi. 55, figs. 1 to 4. 



