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 available
 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