64

4. The fertilizer situation in the United States 100

5. Diagram showing the chemical composition and heat
efficiency of the several ranks of coal 122

6. Origin and development of coal 123

7. Chart showing the present tendency of the United
States in respect to its unmined reserve of petroleum 134

8. The annual output of the principal oil fields of the
United States for the last twenty years 135

9. Curve showing the usual decline in oil field
production after the period of maximum output is
reached 136

10. Chart showing the relative values of the principal
petroleum products manufactured in the United States
from 1899 to 1914 138

11. Alteration of Lake Superior iron formation to iron
ore by the leaching of silica 168

12. Representing in terms of weight the mineralogical
changes in the katamorphism of serpentine rocks to
iron ore, eastern Cuba 172

13. Diagram showing gradation from syenite to bauxite in
terms of volume 245

CHAPTER I

INTRODUCTION

SURVEY OF FIELD

In adapting ourselves to physical environment it has been necessary to
learn something about the earth. Mainly within the last century has this
knowledge been organized into the science of geology, and only within
the last few decades have the complex and increasing demands of modern
civilization required the applications of geology to practical uses,
resulting in the development of the science generally known as _economic
geology_. This science is not sharply marked off from the science of
geology proper; almost any phase of geology may at some time or some
place take on its economic aspect.

The usefulness of economic geology was first recognized in relation to
mineral resources,--and particularly in relation to metallic resources,
their discovery and development,--but the science has been found to have
much wider practical application. The practice of the economic geologist
in recent years has taken on many new phases.

The geologist is called upon to study the geologic features of mineral
de