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The rock salt is also mined from deep shafts. Similar deposits of salt, formed under like conditions, occur in the rocks of later systems down to the present. The salt beds of Texas are Permian, those of Kansas are Permian, and those of Louisiana are Tertiary. THE MISSISSIPPI VALLEY. The heavy near-shore formations of the Silurian in the Appalachian region thin out toward the west.
The Permian period transformed the face of the earth; it lifted the low-lying land into a massive relief, drew mantles of ice over millions of miles of its surface, set volcanoes belching out fire and fumes in many parts, stripped it of its great forests, and slew the overwhelming majority of its animals. On the scale of geological time it may be called a revolution.
We shall see that it is probable that an even higher type of animal, the mammal, was born in the throes of the Permian revolution. But enough has been said in vindication of the phrase which stands at the head of this chapter; and to show how the great Primary age of terrestrial life came to a close.
In outward appearance it was probably something between the salamander and the lizard. It is very probable that some part of the Permian period was the age of the origin of the Protamniotes.
The total available coal in the British coal-fields, at depths not exceeding 4000 feet, and in seams not less than 1 foot thick, is 90,207,285,398 tons, and taking into account seams which may yet become available, lying under the Permian, New Red Sandstone, and other superincumbent strata, this estimate is increased to 146,480,000,000 of tons.
The periods are lesser divisions of the eras. In the Proterozoic, there are two periods, viz.: the Archaean and the Algonkian. The Paleozoic has six periods, viz.: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
But so far is this from being the case in Great Britain, that nowhere have geologists found more difficulty in drawing the line of separation than between the Secondary and Primary series. The obscurity has arisen from the great resemblance in colour and mineral character of the Triassic and Permian red marls and sandstones, and the scarcity and often total absence in them of organic remains.
But as the cold of the Permian yielded to a genial climate and rich vegetation in the course of the Triassic, the reptiles entered upon their memorable development. The amphibia were now definitely ousted from their position of dominance. The increase of the waters had at first favoured them, and we find more than twenty genera, and some very large individuals, of the amphibia in the Triassic.
But mammals have a temperature approaching 100°. We are to believe that one progressive branch of reptiles, which passed through the sieve of natural selection, during the Permian Ice Age, was capable of being adapted to the colder climate.
Taking an average rainfall of 30 inches per annum, and granting that only 10 inches percolate into the rock, the supply of water stored up by the Permian and New Red formations was estimated by the committee to amount to 140,800,000 gallons per square mile per year. This rate would give, for the 10,000 square miles covered by the formations, in Great Britain, 1,408,000,000,000 gallons.
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