228 ECONOMIC GEOLOGY 
interspace is equal in a clay and in a conglomerate of uniform 
spherical pebbles; the interspace is larger in a clay than in a 
conglomerate with sand grains between the pebbles. Inaclay 
the amount of pore-space available for water-storage is high; 
in a sandy conglomerate it may be low. The amount of 
space for water-storage depends also on the denseness of 
the rock. In newly-deposited mud the particles are loosely 
packed and the interstices are occupied by water; when the 
mud is compressed into clay or shale the particles are closer 
together; the bed may be reduced to a sixth of its original 
volume, and its water-storage is greatly reduced. The 
connate water is expelled as the material shrinks. 
The permeability of a rock is its capacity for the entrance 
and passage of water. Clay is impermeable because its 
pore-spaces are so small that the water can only enter them 
by surface-tension. Sand is permeable because water readily 
flows through its wide spaces. Permeable rocks allow water 
both to enter and drain away quickly, and thus wells in them 
may yield large supplies. . 
The imbibition of a rock is its hold on the water in its 
pores. Chalk has a large water-storage, and readily absorbs 
water; but its particles are so minute that they have a 
large internal surface and hold water firmly. Hence though 
a bed of chalk may contain much water, unless a well in it reach 
a water-charged fissure the yield may be small; for owing 
to the high imbibition of chalk little of its water would 
flow into the well. 
The three properties may be illustrated by the following 
comparison :— 
Sandstone . 
Clay . . 
Chalk 
Water-storage. 
Often low 
Large | 
| 
Permeability. 
High’ 
Low 
High 
Imbibition. 
Low 
High | 
Per Cent. of 
Water avail- 
able from 
Wells. 
High 
Nane 
I.0w 
The yield of water from rocks often depends on the joints 
and fissures. The water capacity of many compact igneous