io6 Studies in Forestry [CHAP. v. 



mullein (Verbascum), hawkweed (Hieracium), spurge (Euphorbia] , these 

 last-named genera occurring in various species, and the narrow-leaved 

 meadow grasses. 



The shrubs which occur most frequently on hills and valleys, especially 

 when the soil is fresh, include the following : alder buckthorn (Rhamnus 

 Frangula), blackthorn or sloe (Prunus sptnosa), hawthorn (Cratcegus 

 oxyacantha), spindlewood (Euonymus europ&us), dogwood (Comus 

 sanguined), barberry (Berberis vulgaris), holly (Ilex Aquifolium), honey- 

 suckle (Lonicera Periclymenuwi), elderberry (Sambucus) ; on drier soil, 

 juniper (Juniperus vulgaris}, and on sandy soil, sea-buckthorn (Hippophae 

 rhamnoides). 



Relation to warmth is exhibited in changes of soil-tempera- 

 ture in accordance with changes in the atmospheric tempera- 

 ture. It is determined to a far greater extent by the quantity 

 of moisture in the soil, and by its colour, than by its specific 

 warmth. Retentive soils, like clay, are cold and inactive ; 

 but, when once heated, they only cool down again gradually. 

 Sandy and gravelly soils become easily warmed and stimulated ; 

 but they cool down again rapidly, and hence, in damp localities, 

 increase the danger from frost. In Britain diurnal variations 

 of temperature are obliterated at a depth of about 20 inches 

 from the surface, weekly differences at about 40 inches, and 

 monthly variations at about 6J feet. 



Depth expresses the extent to which decomposition of the 

 soil has taken place below the surface. The finest earth is 

 to be found near the top-level, where the soil has been most 

 exposed to the disintegrating effects of 'weathering'; the 

 deeper one digs below the surface the larger will the stones 

 and breccia be found, until at last a layer of subsoil is struck 

 which is practically beyond the reach of the decomposing 

 agents, and which may, or may not, be of different geological 

 origin from the soil resting upon it. When decomposition 

 has proceeded so far that the ground is easily penetrable by 

 the root-system for a considerable distance below the surface, 

 a soil is termed deep ; but when the decomposed and aerated 

 layer of earth is only slight it is said to be shallow. 



