Where not greatly disturbed by crumpling or upheaval of the earth's crust, the rocks formed in layers are obviously still in their original order, the oldest underneath and the younger layers in order one upon the other, just as they may now be observed in the hills overlooking Dayton, Tenn. Where cut through by rocks which were once in a fluid state, it is apparent that each body of rock is younger than the youngest rock through which it broke and older than the oldest rocks deposited upon its surface after it was solid. Thus the succession of physical events in the history of the earth may be determined by patient and careful scrutiny of the earth's surface as it now is visible, either in natural or artificial exposure such as canyon walls, valley slopes, mines and wells.
In many of these rocks there are found entombed the fossil remains of the animals and plants which were alive at the time the rocks were formed. Some of these are the shells or bones of animals that lived in the seas or lakes, some are the harder parts of animals that lived on the land and were buried beneath the mud of river flats or the ashes blown out of volcanic vents, Discovering these fossil remains and knowing by their physical relations the successive ages of the rocks in which they are found, the geologist is able to sketch the history of animal and plant life upon the earth.
At Least 100 Million Years.
In the very oldest rocks which have yet been discovered, which are at least 100,000,000 years old there are absolutely no traces whatsoever of any animal or plant life. In somewhat younger rocks, but rocks, also referred to the oldest era of geological history, the archeozoic era, there are remains of one-celled plants of the type known as albae. The next era of earth history has been named the proterozoic. In rocks formed during it, there are a very few fossils of lowly types of shell-bearing animals and some rather obscure markings which are probably in part due to the presence of worms and in part represent the remains of sea-weeds. The rocks of these two oldest eras are nearly everywhere much distorted and broken by volcanic activity and crustal upheavals.
Upon these ancient formations there rest in orderly succession the layers deposited during the several periods of time which geologists group into what is called the paleozoic era, which began at least 50,000,000 years ago. Most of the rocks of Tennessee were laid down during that long space of time. In this state, as elsewhere, these strata are known at many places to contain a great abundance of fossils. In the oldest rocks of that era, the fossils are of many and various invertebrate animals, many of which are of kinds not now known to exist anywhere on the face of the earth today. There are no fossils of animals which had a backbone of any sort in any of these rocks. In somewhat younger beds, referred to the second period of the paleozoic era, there are, however, very scanty and fragmentary remains of primitive fishes, the first known animals which possessed a backbone. The oldest known forest, composed of trees of fern-like rather than of seed-bearing types, was found a few years ago in New York in rocks formed about at the middle of this paleozoic era. That was the time when fishes ruled the waters, for remains of sharks and lungfishes are present in great numbers in the rocks formed in the seas, but in the rocks laid down on the land or in swamps there is not a trace of animals with a backbone, although insects and land snails have left their fossil remains in them. Toward the end of the paleozoic era, however, the rocks formed of desert sands and swamps contain the footprints and petrified bones of amphibians and reptiles, the first animals with a backbone which could breathe air by means of lungs. This part of the paleozoic system of rocks includes the coal seams of the eastern states, and associated with the coal are many beautiful specimens of ferns and primitive evergreen trees,
