


366 STRUCTURAL AND FIELD GEOLOGY 
to the neighbourhood, and that, in time, springs and common 
wells may become polluted. This primitive kind of drainage 
is only permissible in new countries where the population is 
sparsely scattered. They should not be tolerated in a 
populous region that is dependent for its water-supply on 
wells and springs, without the most careful inquiry into the 
geological conditions. | 
Low-lying lands with a very gentle slope are often cold 
and wet, or even swampy and boggy. Surface drains in such 
cases may be quite useless, owing to the low gradients. 
Very often it is the presence of an impermeable hard pan 
at the depth of a foot or two below the surface, which 
prevents the escape of the water. With the breaking-up of 
this “ pan,” the superficial water filters into underlying porous 
beds, and the land is at once improved. Occasionally, the 
barrier to the escape of superficial drainage is more deeply 
seated, and may be due to the occurrence of some thick 
impervious stratum of clay, shale, etc., either occupying the 
surface or appearing immediately underneath overlying porous 
beds. In such a case, if no outlet can be obtained by surface- 
draining, it is sometimes possible to sink pits or to bore 
through the impermeable stratum into underlying beds 
which are porous, and through which the superficial water 
eventually drains away. 
Distribution of Disease in Relation to Geological 
Conditions.—Some writers have adduced statistics to show 
that certain diseases are more or less closely associated with 
particular geological systems. This supposed connection 
between groups of rocks and the diseases that afflict the folk 
who reside upon them, is due rather to the physical conditions 
of the regions where those rocks are prevalent than to the 
rocks themselves. The connection, if there be any, is not 
direct, but indirect. In most cases it will be found that the 
more immediate causes of such distribution of disease are 
(2) the water supply, and (6) the physical conditions, more 
especially those which affect the force and direction of the 
winds. Many diseases, as everyone knows, arise from the 
free use of contaminated water. Some water, again, may 
not be polluted by sewage, and yet contain an excess of 
inorganic matter (as lime, magnesia, iron-oxide, etc.) in 

