on direct observation, for man cannot penetrate far below the surface. The deepest mines do not go deep enough to reach materials differing in any essential respect from those visible above ground. Nevertheless, by inference from such observations as can be made, and by repeated and varied experiments in laboratories, imitating as closely as can be devised what may be supposed to be the conditions that exist deep within the globe, some probable conclusions can be drawn even as to the changes that take place in those deeper recesses that lie for ever concealed from our eyes. These conclusions will be stated and the rocks will be described, on the origin of which they appear to throw light.

I have compared the soils and rocks with which geology deals to the records out of which the historian writes the chronicles of a nation. We might vary the simile by likening them to the materials employed in the construction of a great building. It is of course interesting enough to know what kinds of marble, granite, mortar, wood, brass, or iron, have been chosen by an architect. But much more important is it to inquire how these various substances have been grouped together so as to form such a building. In like manner, besides the nature and mode of origin of the various rocks of which the visible and accessible part of the earth consists, we ought to know how these varied substances have been arranged so as to build up what we can see of the terrestrial crust. In short, we should try to trace the architecture of the globe, noting how each variety of rock occupies its own characteristic place, and how they are all grouped and braced together to form the solid framework of the land. This then will be the next subject for consideration.

But in a great historical edifice, like one of the minsters of Europe, for example, there are often several different styles. A student of architecture can detect these distinctions, and by their means can show that a cathedral has not been completed in one age; that it may even have been partially destroyed and rebuilt during successive centuries, only finally taking its present form after many political vicissitudes and changes of architectural taste. Each edifice has thus a separate history, which is recorded by the way the materials have been shaped and put together in the various parts of the masonry. So it is with the architecture of the earth. We have evidence of many demolitions and rebuildings, and the story of their general progress can still be deciphered among the rocks. It is the business of geology to trace out that story, to put all the scattered materials together, and to make known by what a long succession of changes the earth has reached its present state. An outline of what geology has accomplished in this task will form the last and concluding part of this volume.

In the following chapters I wish two principles to be kept steadily in view. In the first place, looking upon geology as the study of the earth's history, we need not at first concern ourselves with any details, save those that may be needed to enable us clearly to understand what the general character and progress of this history have been. In a science which embraces so vast a range as geology, the multiplicity of facts to be examined and remembered may seem at first to be almost overwhelming. But a selection of the essential facts is sufficient to give the learner a clear view of the general principles and conclusions of the science, and to enable him to enter with intelligence and interest into more detailed treatises. In the second place, geology is essentially a science of observation. The facts with which it deals should, as far as possible, be verified by our own personal examination. We should lose no opportunity of seeing with our own eyes the actual progress of the changes which it investigates, and the proofs which it adduces of similar changes in the far past. To do this will lead us into the fields and hills, to the banks of rivers and lakes, and to the shores of the sea. We can hardly take any country walk, indeed, in which with duly observant eye we may not detect either some geological operation in actual progress, or the evidence of one which has now been completed. Having learnt what to look for and how to interpret it when seen, we are as it were gifted with a new sense. Every landscape comes to possess a fresh interest and charm, for we carry about with us everywhere an added power of enjoyment, whether the scenery has long been familiar or presents itself for the first time. I would therefore seek at the outset to impress upon those who propose to read the following pages, that one of the main objects with which this book is written is to foster a habit of observation, and to serve as a guide to what they are themselves to look for, rather than merely to relate what has been seen and determined by others. If they will so learn these lessons, I feel sure that they will never regret the time and labour they may spend over the task.

PART I.

THE MATERIALS FOR THE HISTORY OF

THE EARTH.

CHAPTER II.

THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE EARTH'S SURFACE.

In the history of mankind, no sharp line can be drawn between the events that are happening now or have happened within the last few generations, and those that took place long ago, and which are sometimes, though inaccurately, spoken of as historical. Every people is enacting its history to-day just as fully as it did many centuries ago. The historian recognises this continuity in human progress. He knows that the feelings and aspirations which guided mankind in old times were essentially the same influences that impel them now, and therefore that the wider his knowledge of his fellowmen of the present day, the broader will be his grasp in dealing with the transactions of former generations. So too is it with the history of the earth. That history is in progress now as really as it has ever been, and its events are being recorded in the same way and by the same agents as in the far past. Its continuity has never been broken. Obviously, therefore, if we would explore its records "in the dark backward and abysm of time," we should first make ourselves familiar with the manner in which these records are being written from day to day before our eyes.

In this first Part, attention will accordingly be given to the changes in progress upon the earth at the present time, and to the various ways in which the passing of these changes is chronicled in natural records. We shall watch the actual transaction of geological history, and mark in what way its incidents inscribe themselves on the page of the earth's surface.1 Every day and hour some geological event, trifling and transient or stupendous and durable, comes and goes, leaving sometimes only an imperceptible trace of its passage, at other times graving itself almost imperishably in the annals of the globe. Tracing the origin and development of these geological annals of the present time, we shall best qualify ourselves for deciphering the records of the early revolutions of the planet. We are thus led to study the various chronicles compiled respectively by the air, rain, rivers, springs, glaciers, the sea, plants and animals, volcanoes, and earthquakes-in other words, all the deposits left by the operations of these agents, the scars or other features made by them upon the earth's surface, and all other memorials of geological change. Having learnt how modern deposits are produced, and how they preserve the story of their origin, we shall then be able to group with them the corresponding deposits of earlier times, and to

1 For descriptions of the ordinary operations of geological agents the reader is referred to my Class-Book of Physical Geography. My object now is to direct attention to what is most enduring in these operations, and in what various ways they form permanent geological records.