THE MINERAL FERTILIZERS 199 
theory (W. Newton, Geol. Mag., 1896 ; Singewald and Miller, 
Proc. Soc. Pan-Amer. Sci. Congr., viii, 1917, pp. 873-80) 
attributed the nitrate to bacteria, and its concentration to 
solution by groundwater which washes nitrate out of the soils 
on the Andes and deposits it where the water evaporates 
on the western edge of the pampa. This groundwater 
would also deposit common salt in the saltpans; but, owing 
to its greater solubility the nitrate would be deposited sepa- 
rately on the edge of the salt beds. 
The nitrate has been assigned also to volcanic action by 
the discharge of steam containing ammonia, to lightning dur- 
ing thunderstorms (Semper, 1893, and later Krull), to herds 
of llama and alpaca (O. Kuntze, 1895), and to the bodies of the 
extinct Mastodon and Megatherium (Plagemann, 1905). 
The field relations of the nitrate show that it was deposited 
as an efflorescent salt by the evaporation of groundwater. 
Much of the nitrate may be derived from guano; for the 
salares appear to have been salt lakes that would have 
been frequented by many birds, and the nitrate from their 
droppings would have been dissolved and carried into the 
subsoil after rain or by the floods that occasionally sweep 
over the country from the Andes. The climate appears to 
have been moister during the formation of the nitrate than 
it is now. In dry seasons the nitrate would have been col- 
lected at the surface and deposited on the evaporation of the 
groundwater. Subsequently on the desiccation of the country 
and disappearance of the lakes and birds the supply of guano 
would have ceased, and the beds above the caliche would 
have been cemented by sulphates with occasional patches 
of nitrate. 
The absence of phosphate may appear inconsistent with 
the origin of the nitrate from guano; but as phosphate 
is less soluble it would not have been concentrated in the 
caliche. 
The estimates of the reserves of the material vary from 
70 to 200 million tons. The supply may be expected to 
last, at the present output, for at least 35 to 40 years. Ac- 
cording to Cuevas, it will last for 300 years (Proc. Soc. Pan- 
Amer. Sci. Congr., viii, 1917, p. 62). The industry has been 
seriously affected by the competition of artificial ammonia, 
cyanamide, and calcium nitrate. Moreover, as nitrates are