AGRICULTURAL GEOLOGY 
177 
taining walls and in the erection of dwellings and other buildings. 
A knowledge of the properties and adaptations of structural ma- 
terials is essential to the intelligent selection of them and to their 
efficient use. It is also necessary in many localities to understand 
thoroughly the relations of the substrata to the surface in order 
to choose safe locations for permanent structures. 
The way in which undrained areas were formed has much to 
do with the solution of the problems that arise when drainage is 
undertaken and with the kinds of soil reclaimed when the project 
is completed. Whether an area must be drained by means of sur- 
face ditches or whether an exit may be found through a pervious 
layer of rock below depends wholly on the elevation and on the 
nature and structure of the substrata. In arid and semi-arid 
regions the possibility of irrigation as well as the permanence of 
the aqueduct is dependent also on geologic and topographic fac- 
tors. Of the sewage disposal plants which are needed on all 
farms most types can be located with safety in regard to water 
supply only by considering fully the conditions of geologic struc- 
ture and materials in the vicinity. 
The losses of soil by erosion due to the action of wind or of 
water and in some localities due to the additional influence of 
improper tillage and pasturage bear definite relations to the top- 
ography of the area affected. Unfortunately the rich, black 
humus of the top soil, which is the best part of it, is the first to be 
removed — a fact that makes early prevention imperative. If 
the losses are permitted to continue a great succession of gullies 
and barren ravines soon develops and a worthless area is formed 
where valuable land could have been retained. The water table 
is perceptibly lowered over large areas by increased depth of drain- 
age channels or removal of protective cover and this is another 
serious loss. On the other hand proper drainage may change an 
alkali soil to a fertile one. The chief processes that cause these 
losses involve the principle that the transporting power of running 
water varies as the sixth power of its velocity. This means that 
a current whose velocity is three miles per hour can carry more 
than eleven times as much sediment as one whose velocity is two 
miles per hour and that a current of three miles per hour loaded 
to its capacity will, on being reduced to two miles per hour or 
less, deposit more than 90 per cent of its load. When a flood 
current subsides or is checked, an area of rich soil may be covered 
to a depth of several feet with sand and other worthless material. 
Prevention and partial restoration of losses may be accomplished 
