HEATHER AND SCOTCH FIR 



gives a foot in 10,000 years. By calculation of the weight 

 of the peat formed, Aigner made out that a certain moss 

 was 20,600 years old, and was growing at the rate of two 

 inches in a century. 



But in Denmark ten feet has been formed in 250 to 300 

 years, and in Switzerland three to four feet of peat-moss has 

 been formed in twenty-four years. 



This shows quite distinctly that there is no regular rate of 

 growth, and indeed it is obvious that much must depend on 

 the climate, on the rainfall, on the drainage, and other 

 circumstances. 



Sooner or later, however, a limit comes to the growth of 

 the moss. The surface then becomes gently curved : it is 

 highest in the centre, and slopes very gently down in every 

 direction to the edges. 



What happens next? The first sign is that the surface 

 begins to dry up, and Heather, with grey Cladonia lichens, 

 begins to grow on the projecting tufts and tussocks. 



Occasionally, if gulls build their nests on such drying-up 

 mosses, patches of bright green grass appear wherever the 

 gulls are in the habit of resting. That is due to the lime in 

 their guano. 



But under quite natural conditions a much more important 

 and interesting change begins. 



Here and there scattered over the moss, miserable little 

 seedling Birches and Scotch Firs begin to struggle for life. 

 Of course, if there are hares and rabbits, or if sheep and 

 cattle are allowed to graze upon the moss, those firs have no 

 chance whatever. They are eaten down to the ground. 



But if allowed to go on growing they would no doubt 

 cover the whole moss with a wood of Birch and Scotch Fir. 

 In time that wood would by its roots and its formation of 



358 



