DISADVANTAGES FROM UNFAVOURABLE SOIL, ETC. 51 



consequently at a cheap rate, or not, although, of course, such 

 stone-drains remain active for a much longer period. 



The execution of drainage works by means of the subterraneous 

 pipes, which are so much employed for similar purposes in agri- 

 culture, will, on account of the much greater outlay involved, only 

 very exceptionally find application in woodlands. But at the 

 same time it may be remarked that very wide drain-pipes may 

 here and there be advantageously utilised in road-making in place 

 of the more expensive culverts formed of masonry. 



B. DEFICIENCY OF MOISTURE ; DUNES, SHIFTING SAND, OR 

 SAND-DRIFTS. 



31. Definition, Causes, Disadvantages. 



Sand-drifts are composed of small particles of quartz devoid of 

 all binding or cementing (clayey) material, which, owing to their 

 minuteness and lightness, are apt to be carried from one place to 

 another by the wind ; they are consequently never stationary 

 at one spot, but keep continually moving about. 



Such shifting sands are mostly, and in greatest quantities, to be 

 found skirting the sea-shore and along the banks of many rivers, 

 in consequence of the cementing particles having been washed out 

 by the water and deposited at the bottom, whilst the fine 

 particles of quartz have been thrown on the bank or shore by 

 the waves, and have, in course of time, collected often in enormous 

 quantities (Dunes). 



Shifting sand is, however, also found inland, mostly on areas 

 which formerly formed part of the ocean, and whenever robbed of 

 its soil-covering and its protection, is liable to drift. Such inland 

 sand- drifts are common throughout the great north German plain, 

 stretching from near the Ehine to the eastern confines of Prussia. 



Even when bound together by means of some vegetable soil- 

 covering, such soil is naturally very unremunerative, so that at best 

 I only small returns can be obtained from it by means of timber- 

 production ; but when once the sand is allowed to drift, in con- 

 i sequence of the soil being denuded of its covering, it at once 

 ; threatens neighbouring fertile land, often buries it deep under 

 unproductive sand, and converts it also into sand-drifts, a 

 process that has unfortunately too often taken place over very 

 I considerable areas. 



