TERRESTRIAL BOGS 503 



and the bulrush, Scirpus carspitosus, L., and other species. Thus 

 in the upland bogs of Great Britain, Sphagnum is of little impor- 

 tance as a peat-maker, its place being taken by Scirpus cccspitosiis 

 and Erioplionim z'agiiiatum. Other types, such as the heather, 

 Callnna vulgaris and Erica tctrali.v, are among those dominant in 

 some localities. The Scandinavian and north German moors, on 

 the other hand, are chiefly made up of species of Sphagnum. Where, 

 however, the Sphagnum is exposed during the winter for lack of 

 snow covering, as along the coast of Norway, it disappears, and its 

 place is taken by the bulrush, Scirpus ccrspitosus. Heather peat, 

 formed by Callnna vulgaris, is also found in the high moors of 

 North Germany, as is also Molinia peat, formed by Molinia cavu- 

 Ica. Both Sphagnum peat and peat formed by the Scheuchzeria, 

 the cotton grass and by Scirpus cccspitosus are found in Canada 

 and the northern United States. The place of the heather is taken 

 by Vaccinium (blue berries, cranberries, etc.), Andromeda and 

 other members of the Heath family. 



In northern Germany (Ramann-34: 7jp) the high moors gen- 

 erally consist of an older, strongly decayed Sphagnum peat and a 

 newer, little decomposed, more porous Sphagnum peat, the two be- 

 ing separated by a dividing layer of peat formed by Erioplwrum 

 z'aginatujii and Callnna vulgaris {Erioplioretum and Callunctuni, 

 see the section, Fig. 114). Overlying these is a peat layer formed 

 of the most recent growths, the heaths, lichens, etc. The double 

 character of the Sphagnum peat is explained by the gradual diminu- 

 tion of the water raised in the peat bog by capillarity, so that, as the 

 bog rises by continuous growth, a time will come when the amount 

 of water is too small for further Sphagnum growth. Then other 

 plants take the place of the moss, until by decomposition the latter 

 has been reduced again in volume, and as a result become less 

 permeable to surface waters. Moist conditions will then return, 

 and a new growth of Sphagnum begins. Owing to the fact that 

 the old Roman roads were built upon the dividing layer of terres- 

 trial peat, it becomes possible to measure the rate of growth of the 

 upper peat layer, which must have accumulated during the past two 

 thousand years. 



The external form of the high moor is domed, and permits the 

 run-off of the rain water, which produces moist margins, where 

 hygrophytes of various kinds will grow. 



The upland peat bogs of Great Britain generally occur on un- 

 dulating and sloping ground. When the slopes of the surface do not 

 exceed 10°, the peat generally grows to consideral)le thickness, but 

 where these slopes are greater the peat is also much thinner. 



