THE POLAR GLACIERS. 185 



sorted 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 

 possihle, 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 residt in the later periods was, that the earth was unbal- 

 anced. 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 Oolitic 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 for- 

 mation 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 Sco- 

 tia. 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 be- 

 come 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, alter- 

 nating 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 hemi- 

 sphere to the other, by reason of moderate accumulations of ice in 



