1 20 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



' Ortstein," may frequently be seen in drain-trenches or other ex- 

 posures on our moors. It is a layer of humus-sandstone which 

 may be recognised at the base of the peat if one probes with a long 

 knife-blade downwards from the peat into the soil beneath ; if pan 

 is present, a distinct hard layer is met which turns the knife like a 

 soft sandstone, and when exposed it can be removed as cakes of a 

 dark brown stony material. A form of pan is familiar to farmers 

 on certain classes of soil as a hard layer just beneath the ploughed 

 soil ; its effects are to stunt the root-growth of crops, hence it has 

 to be periodically broken uj) by deep ploughing. Graebner's 

 account of the development of moor-pan is as follows : The re- 

 moval of soluble salts and the accumulation of raw humus lead to 

 the exhaustion of the upper soil, which loses its powdery, crumbly 

 texture, and becomes caked together. The percolation of rain- 

 water carries humus compounds and dissolved salts in solution 

 downwards through the poorer layers of the soil to layers below as 

 yet unexhausted, and here ensues a precipitation of both humus 

 and mineral matter. The process may be seen when a deep trench 

 is freshly dug in a heath area ; water flows in from the peaty soil, 

 and on reaching the bottom of the trench it meets the subsoil 

 richer in mineral matter ; there then occurs a precipitation of a 

 gelatinous brownish layer, which when dry is insoluble in water. 

 Graebner also succeeded in making an incipient pan in a glass tube 

 filled with heath sand. The occurrence of moor-pan in heaths has 

 been proved by borings, and Graebner gives several of these. One 

 locality may be given as an example : {a) heath-peat, 15 cm. (i.e., 

 6 inches); {h) bleisand, 20 cm.; (c) moor-pan, 4-10 cm.; {d) marl- 

 sand, 40 cm.; (e) diluvial sand. This arrangement of the layers is 

 almost constant, though the thickness varies ; the moorpan is 

 always deep enough to be protected from the action of frost, which 

 breaks it when shallow. Chemical analysis of borings shows that 

 below the moor-pan there is always a marked increase in the 

 amount of soluble mineral matter. A well-developed moor-pan may 

 resist the penetration of the roots of young trees or other plants, 

 hence it cuts off the best part of the soil, and limits the plants to 

 the upper impoverished layers. In replanting the heaths of North 

 Germany with forest trees, it is now considered necessary to break 

 up the moor-pan by deep subsoil ploughing, previous to planting. 



Minor causes of disforestation occur locally. Thus the influence 

 of man in digging drains, building reservoirs, and otherwise dis- 

 turbing the existing conditions of drainage, may favour one vegeta- 



