concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of REPLACEMENT. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock. Clays
and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or
shell, frequently forms the nucleus around which the concretion
grows.

Why are building stones more easily worked when "green" than after
their quarry water has dried out?

DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where
ground water is drawn by capillarity to the surface and there
evaporates, it leaves as surface incrustations the minerals held
in solution. White limy incrustations of this nature cover
considerable tracts in northern Mexico. Evaporating beneath the
surface, ground water may deposit a limy cement in beds of loose
sand and gravel. Such firmly cemented layers are not uncommon in
western Kansas and Nebraska, where they are known as "mortar
beds."

THERMAL SPRINGS. While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average
annual temperature of the region, and are then known as thermal
springs. In regions of present or recent volcanic activity, such
as the Yellowstone National Park, we may believe that the heat of