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MR SMITH ON THE CHANGES 



length of time which it must have taken to form the an- 

 cient cliffs and terraces, compared with that which has 

 elapsed since the sea became stationary at its present level. 

 I have since endeavoured to form something like an esti- 

 mate of the duration of these two periods, by compar- 

 ing the extent of the projection of the whin dykes which 

 stand out both from the ancient cliff, and from that which 

 is now forming. I found, upon measuring that part of the 

 dyke which projects from the inland cliff, that it extended 

 208 feet from it, whilst it only extended 1 3 feet from the pre- 

 sent. Hence, it must have taken sixteen times as long to wear 

 away the sandstone in which the dyke has been formerly 

 embedded, in the former period of repose than in the present 

 one. In the annexed diagram, a represents the ancient 

 whin dyke, b the present one, c the former, and d the pre- 

 sent sea-level. 



We have here an opportunity of comparing a minute 

 fraction of geological time with a very long one of histori- 

 cal time, or rather with one of which the whole of histori- 

 cal time is but a part, — probably a small part. 



I am aware that numerical results drawn from a single 

 observation of this nature, must be liable to great errors. 

 But I have had no opportunities of multiplying them since 

 this mode of comparison occurred to me, and I give it mere- 

 ly for the purpose of shewing how such observations may 



