THE COMPOSITION OF THE SOIL 135 



Forbes l recognises three classes of peat in Ireland : 



1. Mountain peat, corresponding with the wet peat low- 

 land moss of the British Committee, which originates wherever 

 the conditions are too sterile or the subsoil too impervious or 

 water-logged to allow deep-rooted vegetation to flourish, and 

 where, therefore, shallow rooted plants come in and, on dying, 

 form a layer of organic matter on which sphagnum, cotton 

 grass, etc., begin to develop. This occurs above the 800 feet 

 level in most parts of Ireland, but in the west it often covers 

 the entire surface down to the sea-level. 



2. Marsh peat, corresponding with the British fen, which 

 arises from reeds, sedges, rushes, etc., and which, so long as 

 the water contains lime and nutrient salts, is as favourable a 

 medium for plant growth as ordinary soil, though it affords no 

 root-hold for trees, so that they are liable to be overturned in 

 strong gales. This kind of peat forms the basis of all the low- 

 land bogs in Ireland and of many of the small bogs in mountain 

 districts. 



3. This marsh peat finally becomes so consolidated with 

 time and pressure that it loses connection with the water table, 

 and a surface swamp forms on which a sphagnum bog of the 

 "mountain type" arises. This, therefore, becomes similar in 

 character to the first group : it differs, however, in its uniform- 

 ity of growth, being higher in the centre than at the margins 

 where soil water can get in and where, therefore, decomposition 

 is more rapid. 



The Scotch peats have been described by Lewis 2 and the 

 Yorkshire moors by Elgee. 3 



Within each of the great classes described above several 

 subdivisions are recognised, but how far they arise from differ- 

 ences in the organic matter, or from other differences, cannot 

 yet be ascertained. 



1 A. C. Forbes, Clare Island Survey, 1914, 9 (Proc. Roy. Irish Acad., 



, 31)- 



2 Trans. Roy. Soc., Edinburgh, 1905, 41, 699-724; 1906, 45, 335-360; 1907, 



3-70 ; 1911, 48, 793-833- 



3 F. Elgee, The Moorland of North-Eastern Yorkshire. London, 1912. 



