Geology: The Science of the Earth's Crust. William J. Miller

      CHAPTER I

      INTRODUCTION

      E

      EARTH features are not fixed. The person of ordinary intelligence, surrounded as he is by a great variety of physical features, is, unless he has devoted some study to the subject, very likely to regard those features as practically unchangeable, and to think that they are now essentially as they were in the beginning of the earth’s history. Some of the most fundamental ideas taught in this book are that the physical features of the earth, as we behold them to-day, represent but a single phase of a very long-continued history; that significant changes are now going on all around us; and that we are able to interpret present-day earth features only by an understanding of earth changes in the past.

      Geology, meaning literally “earth science,” deals with the history of the earth and its inhabitants as revealed in the rocks. The science is very broad in its scope. It treats of the processes by which the earth has been, and is now being, changed; the structure of the earth; the stages through which it has passed; and the evolution of the organisms which have lived upon it.

      Geography deals with the distribution of the earth’s physical features, in their relation to one another, to the life of sea and land, and human life and culture. It is the present and outward expression of geological effects.

      As a result of the work of many able students of geology during the past century and a quarter, it is now well established that our planet has a definitely recorded history of many millions of years, and that during the lapse of those eons, revolutionary changes in earth features have occurred, and also that there has been a vast succession of living things which, from very early times, have gradually passed from simple into more and more complex forms. The physical changes and the organisms of past ages have left abundant evidence of their character, and the study of the rock formations has shown that within them we have a fairly complete record of the earth’s history. Although very much yet remains to be learned about this old earth, it is a remarkable fact that man, through the exercise of his highest faculty, has come to know so much concerning it.

The following words, by the late Professor Barrell, admirably summarize the significance of geological history. "The great lesson taught by the study of the outer crust is that the earth mother, like her children, has attained her present form through ceaseless change, which marks the pulse of life and which shall cease only when her internal forces slumber and the cloudy air and surf-bound ocean no more are moving garments. The flowing landscapes of geologic time may be likened to a kinetoscopic panorama. The scenes transform from age to age, as from act to act; seas and plains and mountains of different types follow and replace each other through time, as the traveler sees them succeed each other in space. At times the drama hastens, and unusual rapidity of geologic action has, in fact, marked those epochs since man has been a spectator upon the earth. Science demonstrates that mountains are transitory forms, but the eye of man through all his lifetime sees no change, and his reason is appalled at the conception of a duration so vast that the milleniums of written history have not accomplished the shifting of even one of the fleeting views which blend into the moving picture."[A]

[A] Central Connecticut in the Geologic Past, pp. 1–2.

      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