Page:Popular Science Monthly Volume 9.djvu/205

Rh drift, and the scratched and polished rocks, all through the stone presentations. But very few, if any, such evidences have been found.

Again, for a warm and exuberant climate to extend into the arctic zone, there was necessary one of those great summers of considerable eccentricity, without the excessive drainage, which an unusually large accumulation of ice in the opposite hemisphere would necessitate. Each summer cycle of coal forests, or of reptile monsters, implies, not only a long visit, and a high evaporating power of the sun, but also the addition, to the opposite polar regions, of a weight of ice only sufficient to draw the waters from a small part of the low and flat lands of the warmer hemisphere. We have seen that periods of warm, perhaps even tropical climates in polar latitudes, intervened between the great winters of the last glacial epoch. But they have left scarcely a trace in the strata. They were the nearest approach possible, with the sun-power of recent times, to the conditions which of old brought out such a profusion of animal and vegetable life. But the only result in the later periods was, that the earth was unbalanced. All the waters were either turned into ice, or were following after it toward one of the poles. One side of the world was a frozen waste, while the other was a burning waste.

I think we cannot avoid the conclusion that the sun shone with a far intenser power on the Carboniferous swamps and the Oölitic shoals than on the gravel-hills of the Drift; that the oceans of early times were wider and warmer than now, and circulated more freely between the tropics and the polar seas; and that the heated and moisture-laden atmosphere retained the heat and equalized the temperature between the equator and the poles far more than at present.

With these conditions, that is, with a greater sun-power and a considerable eccentricity of the earth's orbit, I can conceive a rational explanation, that which I have not yet seen in the books, of the formation of the coal-layers, alternated as they always are with marine deposits. These alternations are sometimes very numerous. There are as many as sixty distinct veins of considerable thickness, one over another, in the coal-mines of South Wales, as also of Nova Scotia. There must have been, in that case, sixty periods of dry land, each of sufficient duration to grow many forests, and each followed by a long-continued submergence, in order that each layer should become fossilized, and buried beneath a shale or a limestone, which could only have formed in the depths of a quiet sea. The books say there were so many upheavals, and a like number of subsidences, alternating with each other. As if Old Earth had bent her back, for her load of pit-coal, threescore times among the Welsh hills, and again as many more at Halifax. It is a far more reasonable explanation, that each considerable layer of coal indicates a cycle of long summers, and the withdrawal of a moderate depth of the oceans from one hemisphere to the other, by reason of moderate accumulations of ice in