A geological chronology is constructed by carefully determining, first of all, the order of superposition of the stratified rocks, and next by learning the fossils characteristic of each group of strata. To many it has seemed that this is reasoning in a circle, but that is because the argument is not fully stated, some of its steps being omitted. The order of succession among the fossils is determined from the order of superposition of the strata in which they occur. When that succession has been thus established, it may be employed as a general standard.

Great physical events, such as the upheaval of mountain ranges, widespread transgressions of the sea and changes of climate, often give us a means of correlating the strata of different continents with greater precision than can be done with the aid of fossils only. The latter are, however, indispensable means of first determining which of these events are comparable in different regions. The history is recorded partly in the nature and structure of the rocks, partly in the fossils, and partly in the topographical forms of the land and the courses of the streams. By combining these different lines of evidence, local histories are constructed for each region, until from these the story of the whole continent may be compiled. The comparative study of the fossils then gives the clew for uniting the history of the different continents into the history of the earth. Much remains to be done before this great task can be accomplished, but already we have an outline of the scheme which future investigations may fill up.

It necessarily follows from the way in which sedimentary rocks are formed, and the local nature of upheavals and depressions of land, that in no single locality can the entire series of strata be observed, and that each region can display but a certain proportion of the whole record. The different parts of our continent are of vastly different geological dates, and even the same area may have been many times, a land-surface, and as often a seabottom. Unconformities, more or less widespread, offer a natural and convenient mode of dividing the strata into groups, but the difficulty with this method is that the dates of elevation and depression so seldom correspond in different regions, that divisions thus made are apt to be of more or less local validity. The only standard yet devised which is applicable to all the world is that founded upon the progress of life.

The comparison between human history and geological history is one that has very often been made, but trite and hackneyed as it is, it is none the less instructive. The history of civilized nations is the record of continuous development, not without retrogressions and periods of comparative stagnation, but having no actual gaps in it. For the sake of convenience, history is divided into certain periods in accordance with the predominance of certain great ideas and principles, and these periods are real, representing the salient facts in the progress of development. Each period is, however, but the outcome of the antecedent periods, and the ideas and principles which characterize it were slowly maturing, it may be through centuries, and even after other ideas have risen to predominance, older ones continue to live and influence the world. For example, when we speak of the age of the French Revolution, we refer to a time when a certain set of political ideas and principles were the most striking and influential factors in the development of the civilized world, beginning with the visible changes of 1789 and ending with the restoration of the Bourbons in 1815. But the tremendous outbreak was slowly preparing throughout the eighteenth century; the conflagration was proportionate to the materials that had been gathered for it.

Nor, on the other hand, could the effects of the great movement be undone by the return of the exiled king. To this day the whole civilized world feels the effects of the convulsion, and the entire course of the nineteenth century would have been different but for the French Revolution.

Historians are careful to distinguish between events and the record of them. Events are continuous and bound up into a chain of consequences, every one of which is dependent upon others, while the records may be scanty, interrupted, confused, unintelligible, even misleading and falsified, so that it is no easy task to write history accurately and without attributing undue importance to this or that principle or policy.

These considerations fully apply to geological history; its divisions are founded upon the rise and culmination of great groups of animals and plants, which one after another have risen to predominance and then declined, their place being taken by others better fitted for the new conditions. These successive culminations are not sudden, but gradual and continuous, and the beginnings of each group are to be found in times long before the period of its predominance. Nor is decline immediately followed by extinc-tion; one group slowly gives way to another, but long after the first has ceased to be the principal fact in the world's life, it may linger on in diminished importance until, perhaps, it finally disappears. The geological periods, therefore, like historical periods, had not definite beginnings and endings, for one slowly fades into another, but they are none the less actual because the lines of separation between them must often be somewhat arbitrarily drawn, and they cannot always be made to correspond in different regions.

In geology, as in history, we must distinguish between the events and the records of them. The more complete the records, the more obviously continuous and gradual was the course of events; only imperfect records can make the history seem broken and disjointed. As our science was first developed in western Europe, where the great groups of strata are mostly separated by unconformities, with abrupt changes in the fossils, the older geologists very naturally concluded that the great divisions of geological time were marked by frightful catastrophes which devastated the earth, destroying every living thing upon it. Each group of rocks was looked upon as the product of a long and tranquil period, and its fossils were believed to represent an entirely new creation. Though opposed by some far-seeing minds, the doctrine of Catastrophism, as it was called, long held sway, but was shown to be erroneous, when the study of geology was carried to other parts of the world. Then it appeared that the supposed catastrophes, if they occurred at all, were not general, but local, and that records missing in Europe had been preserved, partially at least, in other continents.

Enough of these missing records has been recovered to show that the earth's progress was not by a series of abrupt and sudden changes, but by a continuous, orderly development.

Major divisions of geological time are founded upon the more striking changes in the animals and plants, while for minor divisions the more detailed differences in the organisms are employed. Parallel with the divisions of time run the groups or systems of the strata, for characterizing which both the physical nature and structure of the rocks and the fossils are employed. In the very difficult and complicated task of compiling the earth's history, no kind of evidence can be ignored, and wide knowledge and sound judgment are needed in the work, so that no particular class of records shall be either over- or undervalued.

Though the goal of geological inquiries is to construct the history of the earth as a unit, this goal can be reached only by the minute and exhaustive study of the local histories. Each of the latter has certain peculiarities of its own which must be determined, and hence arises the multiplicity of local names for groups of strata, so confusing to the student. Local names are useful, because they avoid the necessity of premature correlations, which may lead to the direst mistakes.