Geology: The Science of the Earth's Crust

by William J. Miller

Or in the words of Tennyson:

There rolls the deep where grew the tree.
O, earth, what changes hast thou seen!
There where the long street roars, hath been
The stillness of the central sea.
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mist, the solid lands,
Like clouds they shape themselves and go.

The following statement of some of the more definite important conclusions regarding earth changes may serve to make still clearer the general scope of the science of geology. The evidences upon which these conclusions are based are discussed in various parts of this book. For untold millions of years the rocks at and near the earth’s surface have been crumbling; streams have been incessantly sawing into the lands; the sea has been eating into continental masses; the winds have been sculpturing desert lands; and, more intermittently and locally, glaciers have plowed through mountain valleys, and even great sheets of ice have spread over considerable portions of continents. Throughout geologic time, the crust of the earth has shown marked instability. Slow upward and downward movements of the lands relative to sea level have been very common, in many cases amounting to even thousands of feet. Various parts of the earth have been notably affected by sudden movements (resulting in earthquakes) along fractures in the outer crust. During millions of years molten materials have, at various times, been forced into the earth’s crust, and in many cases to its surface. Mountain ranges have been brought forth and cut down. The site of the Appalachian Mountains was, millions of years ago, the bottom of a shallow sea. Lakes have come and gone. The Great Lakes have come into existence very recently (geologically), that is to say, since the great Ice Age. A study of stratified rocks of marine origin shows that all, or nearly all, of the earth’s surface has at some time, or times, been covered by sea water. Over certain districts the sea has transgressed and retrogressed repeatedly. Organisms have inhabited the earth for many millions of years. In earlier known geologic time, the plants and animals were comparatively simple and low in the scale of organization, and through the succeeding ages higher and more complex types were gradually evolved until the highly organized forms of the present time, including the human race, were produced.

The rocks of the earth constitute the special field of study for the geologist because they contain the records of events through which the earth and its inhabitants have passed during the millions of years of time until their present conditions have been reached. All the rocks of the earth’s crust may be divided into three great classes: igneous, sedimentary, and metamorphic.

Igneous rocks comprise all those which have ever been in a molten condition, and of these we have the volcanic rocks (for example, lavas), which have cooled at or near the surface; plutonic rocks (for example, granites), which have cooled in great masses at considerable depths below the surface; and the dike rocks which, when molten, have been forced into fissures in the earth’s crust and there cooled.

Sedimentary rocks comprise all those which have been deposited under water, except some wind-blown deposits, and they are nearly always arranged in layers (stratified). Such rocks are called strata. They may be of mechanical origin such as clay or mud which hardens to shale; sand, which consolidates into sandstone; and gravel, which when cemented becomes conglomerate. They may be of organic origin such as limestone, most of which is formed by the accumulation of calcareous shells; flint and chert, which are accumulations of siliceous shells; or coal, which is formed by the accumulation of partly decayed organic matter. Or, finally, they may be formed by chemical precipitation, as beds of salt, gypsum, bog iron ore, etc.

Metamorphic rocks include both sedimentary and igneous rocks which have been notably changed from their original condition. Traces or remains of plants and animals preserved in the rocks are known as fossils. The term originally meant anything dug out of the earth, whether organic or inorganic, but for many years it has been strictly applied to organic remains. Many thousands of species of fossils are known from rocks of all ages except the oldest, and more are constantly being brought to light, but these represent only a small part of the life of past ages because relatively few organic remains were deposited under conditions favorable for preservation in fossil form. The fossils in the rocks are, however, a fair average of the groups of organisms to which they belong. It is really remarkable that such a vast number of fossils are imbedded in the rocks, and from a study of these many fundamental conclusions regarding the history of life on our planet may be drawn.

As early as the fifth century B. C., Xenophanes is said to have observed fossil shells and plants in the rocks of Paros, and to have attributed their presence to incursions of the sea over the land. Herodotus, about a century later, came to a similar conclusion regarding fossil shells in the mountains of Egypt. None of the ancients, however, seemed to have the slightest conception of the significance of fossils as time markers in the history of the earth. (See [discussion below].)

In the Middle Ages, distinguished writers held curious views regarding fossils. Thus Avicenna (980-1037) believed that fossils represented unsuccessful attempts on the part of nature to change inorganic materials into organisms within the earth by a peculiar creative force (vis plastica). About two centuries later, Albertus Magnus held a somewhat similar view. Leonardo da Vinci (1452-1519), the famous artist, architect, and engineer, while engaged in canal building in northern Italy, saw fossils imbedded in the rocks, and concluded that these were the remains of organisms which actually lived in sea water which spread over the region. During the seventeenth and eighteenth centuries, many correctly held that fossils were really of organic origin, but it was commonly taught that all fossils represented remains of organisms of an earlier creation which were buried in the rocks during the great Deluge (Noah’s Flood). William Smith (1769-1839), of England, was, however, the first to recognize the fundamental significance of fossils for determining the relative ages of sedimentary rocks. This discovery laid the foundation for the determination of earth chronology which is of great importance in the study of the history of the earth. (See [discussions below].)