64 VEGETATION OF THE PEAK DISTRICT [CH. 



not continuous under the peat of the southern Pennines, at all 

 events : the birch remains do not exist, for example, on the ex- 

 posed ridges, and they are absent from certain exposed hill sides : 

 they frequently follow the hollows worn out by the streams; 

 and, at their highest limit, they are practically limited to the 

 stream banks. It seems to me unreasonable to elevate these 

 discontinuous birch remains almost to the rank of a geological 

 formation, as is done by some writers on peat ; and it seems best 

 to speak of them simply as the remains of a former birch forest, 

 since their occurrence is exactly what one would expect them to 

 be assuming they are the remains of a thin and open forest 

 which once occurred at the upper local limit of woodland. 



This ancient birch forest is wholly a post-glacial affair ; and 

 the reduction in altitude of the forest limit illustrates what 

 is perhaps a general law that in any district where a forest 

 exists at its extreme limits, climatic or otherwise, the forest 

 will as time goes on exhibit retrogressive tendencies. The 

 latter are usually intensified by human interferences, such as 

 by felling and by the grazing of domestic animals, and, on the 

 other hand, they may be retarded by human interference, as 

 by the careful replanting of the indigenous trees ; but, left to 

 itself, any forest which exists at its climatic or edaphic limits 

 will, in all probability, become degenerate in time. The causes 

 of this degeneration are discussed rather more fully in the next 

 chapter (see page 91). 



It seems to be the case that, in this primitive Pennine 

 birch forest, the Scots pine (Pinus sylvestris) occurred. How- 

 ever, as pine timber is only rarely met with under the peat 

 of the Pennine moors, and as birch timber is abundant, it is 

 impossible to postulate a general zone of pine forest at a 

 different altitude from the birch forest. Probably the pine 

 occurred much more rarely than the birch, either as an 

 occasional associate in the birch association, or it formed smaller 

 associations or societies here and there. On these assumptions, 

 the ancient forest on the upper slopes of the Pennines would 

 be regarded as part of the forest region of north-western 

 Europe, but not, as is the case of the woodlands with birch 

 and pine in southern England, as part of the forest region 

 including the north German plain. The pine probably became 

 extinct here at an early date ; and the existing trees have, in 



