CHANGE OF CLIMATE AND WOODLAND SUCCESSION 249 



woods ; then close-grained. It gives us all the rest of our forest 

 trees, and the whole list of fen-plants associated with limy peat- 

 waters. 



For the Subarctic Period Quercus pedunculata, the Bog-birch, 

 and Scotch Fir, all close-grained, are the characteristic species. 

 For the very lowest level of the fenland all that can be said is 

 that Betula nana is on record without details as to its occurrence. 



This, however, only in the roughest way indicates the nature 

 of our peat records. For instance, there has been no definite 

 Peat Period, though the Middle Period might be so called, from 

 the richness of its species and the greater depth of its varied 

 accumulations. No doubt one time had conditions which pro- 

 duced greater growth than another, but never regularly or for any 

 great length of time without considerable breaks. As many as 

 five "unconformities" or periods of change, marked by renewed 

 forest growth, can be found in one bed of peat only from eight to 

 twelve feet thick. A time conducive to peat growth at one spot 

 would be quite balanced during a less active period by the circum- 

 stances of another place being better adapted for rapid accumula- 

 tion. The whole evidence goes to prove that, from semi-arctic 

 times to the present day, peat has been forming in one place or 

 another. It is impossible to indicate all the recognized facts in a 

 paper. I can only outline the more important ones. 



The old trees of the Middle Period seem to have followed in 

 some such order as this : — 



1. Pinus sylvestris 5. Taxus haccata 



2. Betula tomentosa 6. Acer campestre 



3. Quercus pedunculata 7. Corylus Avellana 



4. Alnus rotundifolia 8. Fagus sylvatica 



Historic Period. 



9. Fraxinus excelsior 10. Ilex Aquifolium 



11. Uhmis montana 



Present Period. 



12. Ulmus campestris 



13. Acer Pseicdoplatanus 



This order may vary very greatly at any given spot, for 

 geological and other reasons. The peat-beds were always formed 

 in low-lying swamps, often apparently by the blocking up of 

 watercourses through wooded ground after a hurricane. Later 

 on, when by changes of drainage the beds dried, or in other cases 

 grew up to the woodland stage level, their tree flora always came 

 from the surrounding higher lands. Now some species can grow 

 freely on peat, others cannot. 



The oak, Scotch pine, bog birch {B, tomentosa), and yew, do 

 well on it. Alder does badly, and only where there is no lime- 

 water, which is rare with us in Lincolnshire, so this tree, even 

 to-day, is rare. The ash and beech grow only where there is a 

 great excess of lime-water, and then onl}? close by the peat, not 

 on it. 



