QUATERNARY DEPOSITS OF BRITISH ISLES — BROOKS. 341 



variable thickness of peat beneath the forest layer, shows that another 

 cause often cited is equally insufficient, namely the upward growth 

 of the bog until it could not longer raise water to its surface by capil- 

 lary action. The only possible explanation seems to be a general 

 climatic change toward drier conditions. There are two other 

 points which support this. The first is that during the forest period 

 the firs in the lowlands attained a size which is not reached by any 

 existing Irish firs, and extended up the mountains to a considerably 

 greater height. I counted over 50 annual rings in a tree stool at 1,200 

 feet on Croghconnellagh, whereas they will now scarcely grow even 

 at sea level in the region ; eA r en greater heights are attained elsewhere. 

 To make this possible the climate must have been considerably warmer 

 as well as drier than now, probably by 3° F. 



The second point which supports the dry-climate theory of the 

 fir forest is the frequent occurrence of trees apparently in situ be- 

 neath the surface of fresh-water loughs. A good example is Lough 

 Toome in southwest Donegal and a small lough to the east of it. 

 There was very little peat on the shores of the lake, and the trees ap- 

 pear to be in situ, though when they grew the water surface must 

 have been at least 2 feet below the level of the present outlet, which, 

 so far as I could see, had never been deeper. Mr. Wynne (1887) has 

 described similar occurrences in Lough Arrow, near Boyle, and in the 

 bed of the Eiver Garwagne, just before it flows over a rock bar. 

 Both localities are in Connaught. He attributes the occurrences to 

 erosion of the peat on which the trees grew, allowing them to settle 

 slowly in the position of growth; this erosion of the peat is 

 still going on. Several cases in northeast Donegal, where the phe- 

 nomenon is very widespread, were investigated by G. H. Kinahan 

 (1886-7). He concluded that the stools are in situ, but that in some 

 cases growth of peat may have choked the drainage, and caused 

 the submergence of the stools, though in others, such as Pollet Lough, 

 Kindrum Lake, and depressions about Lough Aweel there is no ap- 

 parent old exit below the level of the present one. In one of the 

 Lough Aweel depressions bog oak is found at the greatest depth. 

 Most of the loughs in which these submerged stools are found are 

 shallow upland basins with a small drainage area, and in May, 1914, 

 when I saw them, some of them were suffering severely from drought, 

 so that if the present climate became hotter and drier they would 

 more or less completely disappear, allowing trees to grow below the 

 level of the outlet. 



The fir forests were killed by a return of moist and cold conditions, 

 causing a fresh growth of peat. The climate seems to have been more 

 severe than during the earlier period of peat formation, for many 

 hill regions were now peat covered for the first time. On Copped 



65133°— sm 1917 23 



