464 CANNA. DESTRUCTION OF ROCKS. 



these appearing to be confined within limits comparatively 

 narrow. It is evident that similar observations, made in 

 countries where frost is unknown, would be required to 

 confirm this view; but the subject not having heretofore 

 engaged the attention of any one, I am unable to adduce 

 any remarks bearing on the question. If the fact be 

 really such, there will result a great difference in the dura- 

 bility of mountains as well as of sea shores in different 

 parts of the earth ; but it is unnecessary to enter deeper 

 into this speculation. 



At the same time I cannot conclude the subject without 

 remarking that this protecting power of the sea is a fact 

 of considerable importance in marine architecture, and 

 may lead to valuable precautions in cases where the 

 architect has a choice on what foundation his light house 

 or his pier shall be erected. It may perhaps induce him, 

 in cases where he has such a choice, to prefer a submarine 

 basis ; particularly when the rock intended for the build- 

 ing has from its nature a tendency to exfoliate or decom- 

 pose. Had the foundations of the castle and of the walls 

 of Conway been washed by the sea, so many parts of these 

 structures would hot now be suspended in the air and 

 relying on the tenacity of the work. It is satisfactory 

 also to know that the rotten state of the gneiss on which 

 the Eddystone lighthouse is built, presents no cause for 

 fear respecting the permanence of its foundation. 





