and the Distortion of Fossils* 415 



axis is little more than one-fourth of its equatorial axis. The 

 nearly equal amount of compression or extension along the axes 

 a and b, or in the strike and dip of the cleavage planes, is a fact 

 that continually reappears in different districts, and produces a 

 remarkable result in the appearance of the fossils ; viz. that in 

 the plane of cleavage the fossils appear to have undergone little 

 relative distortion, although in some cases, as at Tintagel, they 

 have undergone a considerable extension, preserving their rela- 

 tive normal proportions. 



Example 3. — Carboniferous Slates of Tintagel, Cornwall, 



These slates are somewhat lower down in the geological se- 

 quence than those of Petherwin, from which they differ remark- 

 ably in appearance, being glossy, and of a greenish-yellow colour. 

 Much of this difference may, however, arise from the fact of their 

 being much more cleaved, and retaining traces of having been 

 exposed to much greater pressure. 



The fossils used for the calculation of the amount of distortion 

 were the specimens of Spirifer disjunctus, described by Mr. Sharpe, 

 and now deposited in the Geological Museum in Jermyn Street. 

 I have used in my calculation the normal measurements already 

 given. 



The following are the results obtained from many measure- 

 ments:— 



^=0, — = 1*494 Spirifer disjunctus J 



(ft' = 12°, —=2*514 Spirifer disjunctus. 

 From these data I readily obtain, by equations (4) and (5), — 



a 



i-= 1-494 

 o 



-=9-839 

 c 



We here obtain evidence of an enormous amount of compres- 

 sion perpendicular to the planes of cleavage, being, in fact, rela- 

 tively ten times greater than the compression in the cleavage plane. 

 I should be induced to infer from this result, that the mud or sub- 

 stance from which the rock was formed must have been in a very 

 peculiar condition at the time of compression, perhaps like one 

 of those bulky hydrated precipitates of alumina or peroxide of 

 iron so familiar to the chemist, or like the mud cloud depositing 



(C) 



