494 
GEOLOGY: E. BLACKWELDER 
carbonate shells may at the time be falling from the surface to the 
bottom of the sea. In such a phosphatic deposit the content of phos- 
phorus pentoxide may range from less than one to at least 36%. Sedi- 
ments of this type, subsequently lifted above the sea, have become the 
rare but locally voluminous source of our most important commercial 
phosphates, or have served as a necessary antecedent for the later 
formation of such deposits. 
Reference has already been made to the fact that through the 
agency of birds and other animals of the land, phosphorus may escape 
from the cycle of changes in the ocean. Upon islands where they are 
out of reach of predaceous mammals, sea-birds congregate in extraor- 
dinary numbers, and the amount of excrement annually deposited by 
them on the surface of these islands is large. 
In moist regions bacterial fermentation decomposes this material 
and the soluble resultants are removed by rain water so rapidly that 
little residue is left. On those islands, however, which are situated in 
the dry trade-wind belts, this process is almost inoperative, and hence 
the guano accumulates from year to year. The dry guano — before 
subsequent alterations have taken place — contains as much as 28% 
P2O5, plus organic matter. During moist seasons, transient though 
they may be, the guano is subject to fermentation by microorganisms 
such as the bacteria, with the result that the nitrogenous matter is 
largely converted into ammonia, and is lost. The occasional rains 
dissolve out some of the soluble phosphates and most of the nitrates, 
leaving a residue of solid 'stone guano^ containing as much as 39% of 
P2O5, largely in the form of hydrous acid and basic calcium phosphates. 
The solutions derived from the guano above sink downward through 
the underlying rocks, and produce characteristic alterations in them. 
If the subjacent terrane be limestone, it is slowly converted into a mass 
of calcium phosphate, in which the minerals collophanite and stafel- 
lite (?) are the characteristic forms. Observations on the islands in 
the Indian Ocean indicate that coralline limestone has been changed in 
recent times at the rate of 2 or 3 feet in twenty years. ^ In the labora- 
tory, a coral skeleton became 60% phosphatized in only two months.^ 
On volcanic cones, even such refractory igneous rocks as andesite have 
been converted superficially into a mass of aluminum and iron 
phosphates. 
These processes of alteration near the surface of the earth affect not 
only the guano deposits, but also the marine sediments which contain 
phosphatic minerals. Some of the Cretaceous chalk deposits of Bel- 
gium, which it appears originally contained from 1 to 4% of P2O5, 
have been converted by the differential leaching of lime-carbonate by 
