THE FORESTS AND THEIR PRODUCE. 63 



the south-west with the isotherm of 40 running ahnost north and 

 south through the centre of the islands. The mean temperature 

 of July ranges from 55 degrees in the north of Scotland to 67 in the 

 south-east of England, while the isotherm of 60 passes from west 

 to east through the centre of Ireland and England. The mean 

 annual temperature varies from 47 in the north to 51 in the south. 

 Winter temperatures below 10 degrees and summer temperatures 

 above 90 degrees are uncommon. 



The rainfall is heaviest in the western mountainous districts, 

 showing an average of 50 to 60 inches, with a maximum of 

 200 inches on Snowdon. It decreases gradually to 25 inches on 

 the east coast of England and Scotland, and to about 30 inches on 

 the east coast of Ireland. The rainfall is fairly distributed over 

 the four seasons of the year, though spring is the driest and 

 autumn the wettest season ; drought of sufficient intensity to 

 kill other than newly planted trees is of rare occurrence. 



The relative humidity of the atmosphere is almost always high ; 

 fogs and a cloudy sky are of frequent occurrence. Snow falls in 

 most winters, but, except in the mountains, rarely lies for more 

 than a few days. 



There is a difference between the climate of the east and that 

 of the west, the former being colder and drier than the latter. 

 Certain species coming from a more southern climate do well only 

 in the south-west of England and Ireland ; also, firs and spruces 

 attain a better development in the west, and pines in the east, but 

 otherwise the differences are not sufficient to inhibit in the one 

 region the growth of any tree species which flourishes in the other. 



section ii.— the forests and their produce. 

 1, The Main Types of Forest Growth. 



During the great Ice Age all tree vegetation, with the exception 

 of some arctic forms, was swept away. When the ice receded the 

 trees of historic times came in, and of these the following are 

 considered indigenous : — Scots pine, the common juniper, yew, 

 oak, ash, beech, alder, hazel, birch, cherry, hornbeam, wych elm, 

 holly, field maple, white beam, white willow, black poplar, aspen, 

 and mountain ash, or rowan. 



It has been held that the glacial epoch in Central Europe was 



