15 16 Proceedings of the Royal Irish Academy. 



larger communities for which he employs the name ' formation ' appear to 

 represent groups of associations rather than formations. The term ' forma- 

 tion ' if used to correspond with ' salt-marsh formation,' or ' sand-dune 

 formation ' of phanerogamic botanists, would be more properly applied to the 

 vegetation covering such areas as exposed rocky coasts, or sheltered muddy 

 shores. These might indeed be called ' subformations ' by some botanists, 

 in which ease the whole marine flora would form one formation — the 

 'salt-water formation.' This, however, is hardly justifiable, and there 

 appears to be no reason why the term 'formation' should not be applied 

 to the principal types of the marine vegetation, as well as to the sand- 

 dunes or peat-moors that occur above them. 



In the present state of our knowledge it is difficult to define these 

 marine formations ; but notwithstanding this I have ventured to use the 

 term for the vegetation covering the principal types of ground that occur 

 on our shores. The formations proposed are three in number : — 



1. The rocky-shore formation. 



2. The sand and sandy-mud formation. 



3. The salt-marsh formation. 



All three are represented in the Clare Island area, and in addition two 

 other types are distinguishable, which are dealt with under the heading 

 of— 



4. The vegetation of river-mouths. 



5. The vegetation of brackish bays. 



Each of these five types of vegetation consists of a number of associations, 

 and the latter may frequently show natural groupings. But the associations 

 found vary considerably in different localities, being influenced perhaps 

 most largely by the factor of wave-exposure. 



It should be clearly understood that the classification suggested above 

 makes no pretension to be final. It is possible that the rocky-shore 

 formation will have to be divided into other formations ; but, judging from 

 general field-work in many parts of the British Isles, this does not appear 

 very probable. The flora certainly varies with the nature of the rock, 

 but the changes consist in the modification or re-arrangement of the associa- 

 tions, rather than in any profound alteration in the plant-formation. The 

 general plant-covering of all rocky shores (including chalk) appears to 

 be of the same type ; and for this reason it is here regarded as a simple 

 formation, though further work will doubtless show many differences in 

 detail. 



