446 



Scientific Proceedings, Royal Dublin Society. 



An explanation of this peculiarity — which at all events evi- 

 dently obtains extensively in the Bray Head rock — is not easily 

 offered ; but I think the polarity of the marks respecting the 

 plane of deposition is of importance in this, that it establishes a 

 relation between the phenomena giving rise to them and that 

 plane. Thus, for example, any hypothesis ascribing their origin 

 to something in the nature of crystallization of the materials of 

 the rock must account for a direction of cleavage differing in the 

 two varieties respecting the plane of bedding. This would appear 

 to render a frost-mark theory (these Proceedings, antea, p. 156) in- 

 adequate to explain both forms, although the polarity in the case 

 of the 0. radiata would accord with the theory. On the other hand, 

 it need not necessarily, I think, be opposed to an organic origin 

 for both forms. 



It is observable that if fragments be peeled off the slate, it is 

 often found that the marks have been transmitted, or extend, to 

 layers beneath, so that lines on the upper are seen as continued on 



the adjacent lower surface ; this, too, for thicknesses exceeding a 

 millimetre. 



The accompanying woodcut recalls the appearance of a surface 



