9 4 Proceedings of the Royal Irish Academy. 



The bog areas containing tree-stumps are fairly numerous, and, like the 

 scrub, are confined to the south-east side of the island, but they indicate a 

 much wider extension than the latter now possesses. 



A glance at the map shows that the Pine and Oak remains are found 

 in or beneath the peat over about one-half of the total area of the island, and 

 these are usually mixed with Willow, Birch, and Alder. But while Pine is 

 the predominant species within 100 feet or so of sea-level, Oak becomes more 

 common, and may possibly have been the chief species between 200 and 400 

 feet, few, if any, traces of tree-growth being found above the latter elevation. 

 Before dealing with the significance of this fact, it is necessary to refer to the 

 origin of peat, and see what connexion this deposit bears to the tree-stmnps 

 associated with it. Various theories have been advanced to account for peat- 

 formation, and probably all of them contain a certain amount of truth. It is 

 generally admitted that the first requisite is a cool and damp climate ; but 

 this condition alone would not explain its presence in a large number of 

 cases. The term " peat " usually refers to the partially decomposed remains of 

 various vegetable growths ; but the conditions which have arrested decomposi- 

 tion are not always the same, and it is these conditions, rather than the 

 character of the vegetation, which form the more important differences 

 between various kinds of peat. 



So far as Ireland is concerned, peat may be roughly divided into three 

 classes — mountain, marsh, and high bog peat. Mountain peat originates 

 wherever the surface conditions are too sterile, or the subsoil too impervious 

 or water-logged to allow deep-rooting plants to flourish. The surface is 

 accordingly taken possession of by shallow-rooting plants, which do little to 

 maintain the connexion between the surface aDd subsoil, and a deficiency in 

 the lime content of the soil sooner or later arises. This gives rise to raw 

 humus, and Heather, Bracken, "Wood-rush, Vaccinium, Molinia, Rushes, &c, 

 gradually creep in, and a dry heath peat or sour swamp peat begins to form 

 according to the moisture conditions. Under the heath peat, iron pan invari- 

 ably arises a foot or so beneath the surface sooner or later, and brings about 

 surface stagnation, and on both classes Sphagnum and other water-holding 

 mosses, Cotton-grass, &c, gain a footing, and add to the accumulating mass 

 of vegetable matter. This mountain peat is not confined to the mountains, 

 although the most extensive development of it takes place above the 800-foot 

 level in most parts of Ireland ; but in the west it covers the entire surface 

 down to sea-level in many districts. It varies in thickness from one to ten 

 feet or more, and is more or less mixed up with marsh peat on undulating 

 ground. 



Marsh peat consists of the partially decomposed remains of Eeeds, Sedges, 



