ntary and residual beds described
below.

Phosphorus in the rocks is dissolved in one form or another by the
ground-waters; a part of it is taken up by land plants and animals for
the building of their tissues, and another part goes in solution to the
sea to be taken up by sea plants and animals. In places where the bones
and excrements of land animals or the shells and droppings of sea
animals accumulate, deposits of phosphatic material may be built up.

In certain places where great numbers of sea birds congregate, as on
desert coasts and oceanic islands, guano deposits have been formed. Some
of them, like the worked-out deposits of Peru and Chile, are in arid
climates and have been well preserved. Others, like those of the West
Indies and Oceania, are subjected to the action of occasional rains; and
to a large extent the phosphates have been leached out, carried down,
and reprecipitated, permeating and partially replacing the underlying
limestones. In this way deposits have been formed containing as high as
85 per cent calcium phosphate.

Even more important bodies of phosphates have been produced by the
accumulation of marine animal remains, probably with the aid of joint
chemical, bacterial, and mechanical precipitation. These processes have
formed the chief productive deposits of the world, including those of
the United States, northern Africa, and Russia, and also the phosphatic
iron ores of England and central Europe. The sedimentary features of
many phosphate rocks, particularly their oolitic textures, show a marked
similarity to the features of the Clinton type of iron ores (pp.
166-167).

The marine phosphate beds originally consist principally of calcium
phosphate and calcium carbonate in varying proportions. Depending on the
amount of secondary enrichment, they form two main types of deposits.
The extensive beds of the western United States (in the upper
Carboniferous) are hard, and very little enrichment by weathering has
taken place; they carry in their richer portions 70 to 80 per cent
calcium phosphate, and large sections range only from about 30 to 50 per
cent. In the southeastern deposits (Silurian and Devonian in Tennessee
and Tertiary in the Carolinas and Florida), there has been considerable
enrichment, the rock is softer, and the general grade ranges from 65 to
80 per cent. Both calcium carbonate and calcium phosphate are soluble in
ordinary ground waters, but the carbonate is the more soluble