Glare Island Survey — Tree- Growth. 9 5 



Hushes, and other similar plants which occupy shallow lakes and partially 

 submerged ground. This form of peat constitutes the basis of all the lowland 

 bogs in Ireland, and of a number of small bogs in mountain districts. Marsh 

 peat is closely associated with the existence of drumlins and eskers left by 

 the ice-sheet of the last extensive glaciation of the country, owing to the 

 indefinite drainage system which prevailed for a long period after the ice had 

 retreated. In course of time marsh peat fills up the lake or hollow in which 

 it is accumulating, and the surface raises itself above the water-level. Over 

 this surface, Bog Myrtle, Willow, Birch, Alder, and other terrestrial plants 

 able to bear excessive moisture, gradually spread, and a layer from two to 

 three feet in thickness is gradually built up above the summer water-table. 

 So long as the water contains lime and nutritive salts, this peat forms as 

 favourable a medium for the growth of most plants as ordinary soil, although 

 the nature of the peat prevents tall trees from securing a firm root-hold, and 

 they are liable to be overturned in strong gales. But as the peat consolidates 

 with time and pressure from above, both its capillarity and the percolation 

 of rain-water downwards decrease, and a point is probably reached at which 

 the connexion between the water-table and the surface of the peat entirely 

 disappears, while the soil-water itself may become more acid or diluted as 

 time goes on. 



This condition brings the Sphagnum peat, or high bog, into existence, 

 which differs from the upper surface of the mountain peat more in its mode 

 of origin than in its composition, both types consisting of the same class of 

 plants, and both characterized by a lack of connexion between their surfaces 

 and the soil-water, except where springs or streams happen to traverse the 

 bogs or peat-covered surfaces. "While, however, the Sphagnum peat resting 

 on mountain bog varies in thickness more or less according to the degree of 

 impermeability of the soil-surface, and the conditions favouring the accumula- 

 tion of surface-water, the Sphagnum peat forming the high bogs presents a 

 characteristic uniformity of growth. This consists in a gradual decrease in 

 thickness from the centre of the bogs towards the margins, giving their 

 surfaces a convexity which is so universal that the term " high " bog has 

 been applied to this particular type to distinguish it from the flat or marsh 

 bog which follows or is parallel to the water-table. This convexity is usually 

 attributed to the high power the Sphagnum possesses of holding water by 

 capillarity, enabling the centre of the bog to rise above the level of the 

 margins, where natural drainage outlets prevent a further rise. But it is 

 also probable that the lower level of the margins of a high bog is due to 

 the partial decomposition of the peat when it comes within the influence of 

 the soil-water. It is a well-observed fact that peat decays rapidly when 



