Fig. 3. 





33 Mr. H. C. Sorby on Slaty Cleavage, 



greatest amount of compression. In uncleaved limestones the 

 joints of encrinites have their longer axes either arranged pro- 

 miscuously, or if anything they lie in the plane of stratification, 

 but in this highly cleaved specimen they are very greatly com- 

 pressed in the plane of cleavage. Their forms are quite distinct 

 from those found in uncleaved limestones ; they are often not 

 symmetrical, but broken up irregularly ; and, instead of being 

 on an average nearly equiaxed, their greatest length, which 

 always very closely coincides with the line of cleavage, is on an 

 average about four times that in a direction perpendicular to it ; 

 and they show forms such as would result from the compression 

 and distortion of the sections of the short cylinders, seen in the 

 uncleaved specimens. 



But besides their form being thus changed, 

 the ultimate organic structure is altered in a 

 corresponding manner. When not compressed, 

 the structure of an encriuite joint is as shown 

 in fig. 3, drawn from a fragment in a Devonian 

 limestone, magnified 200 linear. It is cellular, 

 the cells being somewhat angular, varying a 

 little in size, but on the whole nearly equiaxed. This is the 

 character when seen in a section cut perpendicular to the axes 

 of the joints, and in many cases it is very nearly the same in every 

 position; but in others the cells are arranged one over the other 

 inthelineof the axes of the joints, and the walls separating them 

 in that direction are more or less absorbed, so as to give rise to 

 a more or less irregular or perfect tubular structure. In some 

 cases the cells are filled with dark material, whilst their walls 

 are clear and transparent ; in others they are filled with clear, 

 crystalline calcareous spar and the walls are dark, so that the 

 structure may be very readily seen ; but when both are equally 

 clear and transparent or uniformly dark, it cannot be recognized. 



In the highly cleaved limestone from Kingskerswell the struc- 

 ture of the joints of the encrinites is very different from that 

 just described, being usually as shown in fig. 4. This Fig. 4. 

 has quite a difierent character from what is seen when 

 the tubular structure is cut obliquely ; and besides, in 

 uncleaved limestones the tubes in the difi^erent detached 

 joints do not lie all in one direction, but promiscuously, 

 whereas in this limestone the longer axes of the cells 

 lie in the line of the compression of the joints, and 

 nearly always in the plane of cleavage, those that do 

 not quite coincide with it, differing from it only in such 

 a manner as would result from irregular giving way, or 

 from the compression of a tubular structure inclined to 

 the line of cleavage. On an average the cells have their 



