Smyth — Problematic Structure in the Oldhamia Rocks of Bray Head. 231 



account for distortion of angles.' Pyrites suggests itself as containing iron, 

 having a cubic form, and being common in argillaceous sediments. But, uiiless 

 some orienting influence can be assumed, one would expect more varied shapes 

 to result from the compression of cubes. 



EXPLANATION OF PLATE XII. 



.Figs. 1 & 2. — Portions of the bedding plane of the shale, photographed wet. 

 X2-7. 



Fig. 3. — A portion, photographed dry. Oldhamia antiqua on the right ; worm 

 burrow below. Note the slight differentiation of the marks when not 

 wetted. X 1-7. 



Figs. 4 & 5. — Transverse sections through the markings. Each figure contains 

 two of the marks ; the one on the right in flg. 4 is the same as that on the 

 left in fig. 5. X 12. 



Fig. 6. — Camera lucida drawings of some of the spots from various parts of the 

 specimen. X 5. 



Fig. 7. — Camera lucida drawings of pseudomorphs of halite in Triassic marl. 



xrs. 



' That such distortion may be remarkably small was shown by experiments vAih plasticine. 

 Cubes of red plasticine were cut and embedded in green plasticine, the latter being carefully 

 built up round the cubes in order to secure as perfect contact as fiossible. The whole was 

 then strongly compressed between two parallel boards, and sections made in various 

 directions. 



