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XXXVIL— ON A PECULIAKITY IN THE NATUEE OF THE 

 IMPEESSIONS OF OLDHAMIA ANTIQUA AND 

 0. RADIATA. By J. JOLY, B.E., Assistant to the 

 Professor of Civil Engineering, Trinity College, Dublin, 



[Read, November 17, 1886.] 



Recently, while examining some fragments of slate from Bray 

 Head showing marks of Oldhamia, I noticed that on such speci- 

 mens as displayed both varieties of marking, 0. antiqua and 

 0. radiata, the following peculiarity appeared : — A sunken or de- 

 pressed delineation of one variety accompanied a raised or relieved 

 delineation of the other variety. Thus, if on any specimen 

 0. antiqua appeared as a depression, on that same surface the 

 0. radiata appeared in relief. I verified this relation over such 

 specimens as were in my possession, twelve in number, collected by 

 myself at various times from certainly not less than two distinct 

 localities on Bray Head : one of which is the well-known locality 

 close to the Periwinkle Rocks. These specimens are on the table. 



From this observation it appeared probable, if any meaning 

 was to be attached to the relation, that a further relation would be 

 found to obtain between the mode of delineation and the position 

 in the rock. This was easy of investigation, as such further rela- 

 tion might be sought for wherever either variety of mark was to 

 be found in situ. Examination revealed the expected relation, 

 in this order: — On the upper surface, or what was most probably 

 the surface of deposition (the cleavage of the Cambrian slate of 

 Bray Head coincides generally with the plane of bedding), the 

 0. radiata appeared invariably as a depression, the 0. antiqua in re- 

 lief. In four localities this was verified. In one only, at the south 

 entrance to the new tunnel, was there any doubt. Here the folding 

 is so extensive and complicated that it was uncertain what surface 

 was uppermost, and the marks also were obscure. Close to this 

 fifth locality clearer marks on less contorted beds are in accord with 

 the relation. It is apparent, indeed, that in the event of the relation 

 being more extensively verified, it might in such cases be applied 

 to determine whether or not inversion had occurred. 



