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to expect there will also be epochs in this progress, as in human history, which constitute natural subdivisions of the grand subject. This we find to be the case; but the author justly cautions us that we must not "expect to find strongly-drawn lines between the ages, nor the corresponding subdivisions of the rocks." It is comparatively easy, as a general thing, to determine the culminating or central point of a period, but it is not so easy to determine the exact beginning or end. Indeed, in the progress of the geological ages or periods, the characteristics of any one period are usually, upon examination, found to have had their origin in the middle of the period preceding. In the continual flow of events, the transition from one condition to another are so easy that the change is scarcely noticed; but when the eye is thrown back it is seen that the characteristics of the present period were really foreshadowed far back in the past.
The rocks formed in any region in successive geological periods, are often found to differ greatly in character; but it is by the progress of life that "geological time derives its division into ages." The rocks change in character as we ascend in the series in any particular locality, and the order of superposition there, as a general rule, will determine their relative age; but how shall the relative age of strata be determined in places remote from each other, and between which no connection can be traced? It is only or chiefly by the fossils found in them, which are themselves the records of the progress of life at the time these strata were deposited. It is not to be supposed that exactly the same geological changes were taking place at the same time in countries distant from each other, but it has been fully determined that the progress of life, in its grand features, has been essentially the same everywhere. And the reason of this, in part at least, the progress of science has made known
One of the chief circumstances affecting both vegetable and animal life, and especially the latter, is the condition of the atmosphere; and this in all its important features must be the same over the whole earth. At present, in every country, and everywhere over the ocean as well as on the land, so far as our observations have extended, the atmosphere is made up of *Dana's Address at Providence, p. 5.
nitrogen about seventy-nine parts, oxygen twenty or twentyone parts, and carbonic acid gas less than one part. The proportional quantity of the latter, however, varies considerably in different places, and at different times in the same place. Occasionally other substances are present, but only in very minute quantities.
But in the early geological periods the constitution of the atmosphere must have been essentially different from this, as a very considerable part of the carbon which we now find beneath the soil, in the form of mineral coal, was then in combination with a part of the free oxygen of the present atmosphere, as carbonic acid. This latter must then have formed a part of the atmosphere, which of course contained proportionally a much larger quantity of carbonic acid, and much less oxygen than the present atmosphere. Other gases also may then have been present, but we have no means now of determining this point. No animals or plants of the present day, especially the former, could have lived at that time, nor could the animals or plants of that period now flourish.
This change in the atmosphere was not made in a day or a year, but being the result of the operation of natural causes, was in progress through a long period of time. And as the change gradually progressed there would necessarily be corresponding changes in both the animals and plants of that period; and moreover as the progressive changes in the atmosphere would extend to all countries on the face of the globe, so would the corresponding changes in the progress of life, as far as affected by this cause. Therefore, though the races of animals inhabiting distant countries at any epoch may have been very different, still a sufficient resemblance would be preserved to identify them as belonging to the same period. The same remark will apply also to the plants of any period, though perhaps with less apparent force.
In every work on geology reference must frequently be made to particular localities in illustration of principles advanced, but our author's method of treating the subject requires more than this. There is no such thing as history in the abstract; a history must be the history of some country or people or individual. The work does indeed treat of the general subject of geology, but the great globe is made up of many parts, each of
which has had its own development, and in a grand measure its own separate history. As in civil history, the history of the world is made up of the histories of the multitudinous nations that have at different periods occupied various portions of its surface, so in geological history, the whole is also made up of its separate parts, which are more or less distinct.
The author in his Preface gives us his reasons for the American character of his work. They are two: "a desire to adapt it to the wants of American students, and a belief that, on account of a peculiar simplicity and unity, American geological history affords the best basis for a text-book of the science. North America stands alone in the ocean, a simple isolated specimen of a continent, (even South America lying to the eastward of its meridians,) and the laws of progress have been undisturbed by the conflicting movements of other lands." The work therefore is designed to be a history of the development of the North American continent in particular, but references are occasionally made to the cotemporaneous geological history of other continents; so, also, localities in other countries are constantly referred to as illustrating important principles, or proving the operation of particular agencies.
Thus the work is made, to a considerable extent, a general discussion of geological history, while it, in a special manner, develops the geological history of our own continent.
In his survey of general geological history, the author recognizes seven periods or ages, each of which is characterized by some few prevailing forms of animal or vegetable life. These ages are not "strongly marked off in the rocks," because "it is not in the nature of history to be divided off by visible embankments;" but still the reality of their existence is easily seen, even when the beginning or ending of an age it may be quite impossible to determine. These ages are,
Firstly, The Azoic Age, so named because in the rocks of that period no traces of animal life are found, and only a few of the lowest orders of plants; they are considered the oldest rocks that meet our view.
Secondly, The Age of Mollusks, during which were deposited the oldest rocks that contain the remains of animals and plants. These rocks, with others of later periods, are called the Paleozoic (ancient life-bearing) rocks. In this age mollusks were
more abundant than any other form of life, but other forms were known. These rocks are also known as the Silurian rocks.
Thirdly, The Age of Fishes, in which fishes were especially abundant. The rocks of this period are also known as the Devonian rocks.
Fourthly, The Carboniferous Age or Coal Period, during which the immense deposits of mineral coal were formed.
Fifthly, The Age of Reptiles, so called because characterized by the existence of huge reptiles, as the ichthyosaurus, the iguanodon, etc., found in England and elsewhere.
Sixthly, The Age of Mammals.
Seventhly, The Age of Man.
This subdivision of geological history has reference solely to the animal kingdom, as will be seen at a glance; but the author remarks that a classification made in reference to the vegetable kingdom would not differ widely from it.
It would be interesting to give here some extracts from the author's "History," to show how very satisfactorily he depicts the probable phases of this continent at different periods as it gradually emerged from the universal ocean, and progressed from stage to stage through the long, long, slowly-creeping centuries; race after race of both animals and plants appearing and disappearing in long succession; until at length it put on its present verdure, and became peopled with its present inhabitants, but it is impossible to do so in our brief space. Only a single extract can be given to show the point of beginning:
The fact of the existence of the globe at one time in a state of universal fusion, is placed beyond a reasonable doubt. And whatever events occurred upon the globe from the era of the elevated temperature necessary to fusion, down to the time when the climate and waters had become fitted for animal life, are events in the Azoic age. The age, therefore, must stand as the first in geological history, whether science can point out unquestionably the rocks of that age or not.-P. 134.
But we must not allow ourselves to be led astray here; as is implied in this extract, we are not always to infer that rocks belong to this age or period because they are destitute of organic remains. The granitic and metamorphic rocks of New England contain no fossils, but it is considered as well settled that they are even more recent than the fossiliferous rocks of north
ern New York. In close proximity to the granite, in some places in Massachusetts, rocks are found containing fossils similar to those in the rocks of northern New York; a fact which indicates that the intruded granite is the more recent of the two.
The development of this continent, according to the author of this work, began with the immense range of granitic or azoic rocks, which extends from a point north-west of Lake Superior in two branches, one to the north-west, parallel with the Pacific coast, and the other to the north-east, parallel with the river St. Lawrence, about midway between that river and Hudson's Bay. These are the true primary rocks of the continent, having so early emerged from beneath the waters that in all probability they were never covered with any sedimentary deposits. Rocks of the same character, but only of very limited extent, are found in some other places in the interior of the continent, as in Michigan, a small distance south of Lake Superior, and perhaps some points in the Rocky Mountain range. These peaks then of course constituted islands in the vast sea. But though only this small part of the continent had at this time emerged above the surface of the ocean, still the great outlines of the continent, as it afterward became developed, were probably well defined, and the whole, with the exceptions named, lay at no great depth beneath the surface of the wide extended ocean.
Thus, the azoic period closed when the surface and the atmosphere had attained a temperature which permitted the introduction of organic bodies, and some of the lower orders, both of plants and animals, made their appearance; and their remains constitute the fossils now found in the earliest paleozoic rocks, the rocks of the next succeeding period.
During the azoic period all the events that occurred were simply physical; but on the introduction of organic bodies, a new order of things was presented, which constituted a new and great step of progress." And this progression thus inaugurated was to continue through many intermediate stages until it should terminate in the "creation of Man and Mind, as the last and loftiest of these culminations."
But we may not follow the author further as he continues his descriptions of the successive developments of this conti