GEOLOGY IN RURAL WELFARE 
285 
coal and other mineral fuels, the mineral oils (kerosene and gaso¬ 
line), iron, salt, gypsum, lime, the minerals of the soil, and the 
fertilizer minerals yielding potash, phosphates and nitrates. The 
nature, quality, distribution and availability of most of these sub¬ 
stances bear direct relations to their respective geological occur¬ 
rences. In order that careful discriminations may be made in their 
purchase and use, those who have need for them should be familiar 
with their distinguishing properties and with their relative values. 
In road building the adaptation of various materials, even when 
only sand and clay are needed, is determined by the properties of 
the minerals and rocks considered for this purpose, and by the 
nature of the base on which the road is to be constructed. In lo¬ 
cating a road along or near a slope, or in any topographic position 
where strata outcrop, the drainage and therefore the safety and 
permanence of the road, or its failure, depends on the kinds of rock 
involved and on their structural relations. The rapidly growing 
use of motor vehicles emphasizes the importance of details in re¬ 
gard to road materials and road locations. 
From the rocks at the surface or below it, suitable material is 
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obtained for buildings and other structures necessary in agricul¬ 
tural enterprises. Such materials are used in making brick, 
cement and concrete, in building roads, bridges, dams and re¬ 
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 rise 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 previous 
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 on geologic and topographic fac¬ 
tors. Of the sewage disposal plants which are needed on all 
