1846.] SHARPE ON SLATY CLEAVAGE. 79 



gibbous shells, as in the latter the parts of the shell lie in different 

 directions towards the plane of cleavage. 



There are some preliminary points to be mentioned. Among the 

 palaeozoic fossils we never meet with the real shell, and seldom with 

 anything representing its substance ; we have only impressions and 

 casts, sometimes with a hollow space showing where the shell has 

 been ; but frequently this space has been filled up by pressure, and 

 we find on splitting the rock one impression which represents both 

 the inside and the outside of the valve ; and among the most dis- 

 torted fossils there is rarely any appearance of fracture. It must 

 therefore be considered that we are reasoning upon such impressions 

 as are made on wax, of which the form may be altered without 

 breaking it by drawing out or compressing the mass. 



In all the arguments drawn from the present forms of the distorted 

 shells, it will be assumed that the original form of the shell is known 

 and its original size unknown ; and constant use will be made of the 

 alteration of the markings on the shell in estimating the nature and 

 direction of the distortion; among these markings the ribs of the 

 Spirifers are most useful, from the well-known symmetry of their 

 arrangement. If we had to deal with the changes in the forms in 

 smooth bodies, of which we know only the original form but not 

 the original size, we should find it impossible to analyse the move- 

 ments which had produced the change ; for an impression of a flat 

 circular form might be changed into an oval by the shortening of 

 one of its diameters, by the lengthening of the other diameter, or by 

 a combination of both causes ; either of the three explanations 

 would be equally possible: but if the circular impression had been 

 crossed by symmetrical curves, of which the direction and relative 

 distances were known, the alteration in these lines would betray the 

 nature of the change. The same will hold true of a solid. 



Let us now proceed to the description of the distorted shells, 

 beginning with those found in beds intersected by the cleavage at a 

 high angle. The change is the simplest where the shells are flat, 

 and therefore lie entirely in the plane of the bedding ; in these cases 

 the shells all appear contracted in a direction perpendicular to the 

 strike of the cleavage across the bed, and this apparently without 

 any lengthening of the shell in the contrary direction, although it is 

 difficult to be sure of this latter point. This change is shown in 

 fig. 3, where several nearly flat brachyopodous shells are lying in 

 various directions on a slab of slaty rock from Aber-y-Wynant, near 

 Dolgelly : the apex of each is marked A, the direction of the clea- 

 vage across the bed is shown by the lines Z Z. Every shell appears 

 shortened in a direction contrary to that of the lines Z Z, as if the 

 whole mass had been pressed together in one direction. The shells 

 probably belong to one of the concentrically marked Atrypae, but 

 the longer forms might be mistaken for Lingulae. The surface of 

 each is covered with small wrinkles, parallel to the strike of the 

 cleavage Z Z ; in this instance the cleavage cuts the bedding at an 

 angle of about 40° : not having the original form of the shell as a 

 guide, we cannot calculate the amount of distortion accurately, but 



