are used by the iron furnaces of this section. The rocks of this age are known as the Silurian, and during this time life further developed and scorpions and lung fishes came into existence.
The series goes on. Layer after layer of rocks were laid down, each series of which has been given a name by geologists so that they can be easily referred to. Next came the great age of fishes, and their remains are found in the rocks which the geologists call the Devonian and Mississippian series. The black slate, which crops out at the foot of Walden's ridge, as well as the limestone lying above it, which form the side of the mountain to the west of Dayton, are layers belonging to these series. These rocks are full of the remains of animal life.
Then came the period in which the ancient plants flourished and produced great coal deposits, the age which has ben called the carboniferous. The extensive coal deposits of the Tennessee coal field, the edge of which caps the mountain a few miles west of Dayton, are of this age, and wonderfully preserved plant remains are found in the slates which lie on top of the different coal seams. This is a fact well known by the coal miners of this section. And what has been stated above as to Tennessee is but one illustration of how the different geologic periods passed and life developed over the earth.
And even when this carboniferous period in the development of the earth has been reached, we are still many millions of years back from the age of man; we must still pass through many geological time periods, through that age known as the Permian, when land vertebrates first arose; through the Triassic, when reptilian mammals arose; through the Jurassic, when flying reptiles were in existence. This was the age of reptiles. Then into the Cretaceous when flowering plants came into existence, and a great group of the reptiles known as dinosaurs, became extinct. And then we came to that period in the earth's history, at the beginning of which the ancient mammals and birds were first known to exist. Fossil remains show clearly that birds evolved from flying reptiles. This is the great age of mammals. Thru this period, the modern life forms developed. A period of glacial activity took place, during which five distinct glacial stages existed, one after the other, with four interglacial intervals, and man-like beings came into being at least the beginning of this time. Such, very briefly, is an account of the evolution of the earth from Cambrian time to the present, with a brief outline of the life forms which existed during these different periods. We know that this took many millions of years, and yet we also know that the earth existed untold millions of years before Cambrian time.
For the formation of the earth and its early stages we must turn to the science of astronomy. The relations of the earth to the stars and the planets are shown in the depths of the leavens, and there must exist in the heavens those cosmic conditions which gave rise to our world and the other planets of our system. Through the telescope and spectroscope, the astronomers have solved many of these secrets.
But what of the age of the earth measured in years as we measure other happenings. From the brief outline just given one can see that it has been in existence unknown millions of years, but just how many it is impossible to say.
We can, however, measure back to the more recent events in geological time to the last ice age, before which we know man existed, and get a fairly accurate result, in terms of years.
One of the most accurate ways in which to measure such time intervals, is by measuring and counting the light colored and dark colored bands of clay, deposited by the melting of the ice sheet in the fresh water lakes which existed on the edges of those continental glaciers, as it retreated to its present position in the north polar regions. Each dark layer of clay was laid down
