sub-capillary in size. Most rocks, even dense
igneous rocks, are porous in some degree, and certain rocks are porous
in a very high degree. The voids in some surface materials may amount to
84 per cent of the total volume. In general the largest and most
continuous openings are near the surface,--where rocks on the whole are
more largely of the sedimentary type and are more fractured,
disintegrated, and decomposed, than they are deep within the earth. The
largest supplies of water are in the unconsolidated sediments. The water
in igneous and other dense rocks is ordinarily in more limited quantity.

APPROXIMATE QUANTITY OF WATER WHICH WILL BE ABSORBED BY SOILS AND
ROCKS[1]

------------------------------------------------------
_Volume of water asborbed
_Material_ per 100 of material_
------------------------------------------------------
Sandy soil[2] 45.4
Chalk soil[2] 49.5
Clay[2] 50-52.7
Loam[2] 45.1-60.1
Garden earth [2] 69.0
Coarse sand [2] 39.4
Peat subsoil[2] 84.0
Sand 30-40
Sandstone 5-20
Limestone and dolomite 1-8
Chalk 6-27
Granite 03.-.8
------------------------------------------------------
1: Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co.,
New York, 1919, p. 393.

2: Woodward, H. B., _Geology of soils and substrata_:
Edward Arnold, London, 1912.

Immediately at the surface, the openings of rocks may not be filled with
water; but below the surface, at distances varying with climatic and
topographic conditions, the water saturates the openings of the rocks
and forms what is sometimes called the _zone of saturation_ or the _sea
of underground water_. The top surface of this zone is called the _water
table_, or the _ground-water level_. The space between the water table
and the earth's surface is sometimes referred to as the _vadose zone_ or
the _zone of weathering_, since it is the belt in which weathering
processes are most active. The zone of weathering is not necessarily
dry. Water from the surface enters and sinks through it and water also
rises through it from below; it may contain suspended pockets of water
surrounded by dry rocks; it i