The writer's indebtedness for information derived from the printed page and for personal discussion and advice is of wide range. He would express his warm appreciation of the friendly spirit of coöperation and advice with which this effort has been aided--a spirit which he likes to think is particularly characteristic of the profession of economic geology. In particular he would acknowledge the efficient aid of Mr. Julian D. Conover in preparation and revision of the manuscript.
THE COMMON ELEMENTS, MINERALS, AND ROCKS OF THE EARTH AND THEIR ORIGINS
A list of the solid substances of the earth making up the so-called lithosphere (or rock sphere) in order of their abundance, does not at all correspond to a list made in order of commercial importance. Some of the most valuable substances constitute such a small proportion of the total mass of the lithosphere that they hardly figure at all in a table of the common substances.
RELATIVE ABUNDANCE OF THE PRINCIPAL ELEMENTS OF THE LITHOSPHERE
When reduced to the simplest terms of elements the outer ten miles of the lithosphere consists of:[1]
PERCENTAGE OF PRINCIPAL ELEMENTS IN THE LITHOSPHERE
Oxygen 47.33 Silicon 27.74 Aluminum 7.85 Iron 4.50 Calcium 3.47 Magnesium 2.24 Sodium 2.46 Potassium 2.46 ----- 98.05
The remainder of the elements exist in quantities of less than 1 per cent. None of these principal elements occur separately in nature and none of them are mined as elements for economic purposes.
RELATIVE ABUNDANCE OF THE PRINCIPAL MINERALS OF THE LITHOSPHERE
Minerals exceptionally consist of single elements, but ordinarily are combinations of two or more elements; for instance, quartz consists of a chemical combination of silicon and oxygen. The proportions of the common minerals in the outer ten miles of the lithosphere are in round numbers as follows:
PERCENTAGE OF COMMON MINERALS IN LITHOSPHERE
Feldspar 49 Quartz 21 Augite, hornblende, and olivine 15 Mica 8 Magnetite 3 Titanite and ilmenite 1 Kaolin, limonite, hematite, dolomite, calcite, chlorite, etc. 3 --- 100
In making up this table it is assumed that the rocks to a depth of ten miles are about 95 per cent of igneous type, that is, crystallized from molten magma, and about 5 per cent of sedimentary type, that is, formed from the weathering and erosion of igneous rocks or preëxisting sediments, and deposited in beds or layers, either by water or by air (see pp. 16-17).