Br. James Croll — On Geological Time. 397 



agencies so extraordinary and gigantic? To carve a country, say like 

 Scotland, out of hard Silurian rock into liill and dale and mountain 

 ridges, thousands of feet in height, is certainly a more stupendous 

 undertaking than simply to cover the same area with a sheet of ice. 

 And if commonplace agencies like rain and rivers, frost and snow, 

 can do the former, why may not such agencies as ocean currents, 

 winds, clouds, and aqueous vapour, be sufficient for the latter ? 



That geological climate should depend on the causes to which we 

 refer cannot appear more improbable to the geologists of the present 

 day than the inference that hills and valleys were formed by atmo- 

 spheric agencies did to the geologists of the last generation. And 

 there is little doubt that by the next generation the one conclusion 

 will be as freely admitted as the other. 



When a physicist so eminent as Sir William Thomson expresses 

 his decided opinion that the agencies in question are all that are 

 necessary to remove the ice from the Arctic regions, and confer on 

 them a mild and temperate climate, it is to be hoped that the day is 

 not far distant when the climate controversy will be concluded. 

 When the fact comes to be generally admitted by physicists that a 

 great increase in the temperature and volume of the ocean currents 

 flowing polewards is sufficient to prevent the accumulation of ice in 

 the Arctic regions, it will then be allowed that we only require a 

 great decrease in the volume and temperature of the currents in order 

 to account for the former accumulations of ice on the temperate 

 regions, or, in other words, to explain the occurrence of the Glacial 

 Epoch. And when this position is reached, it will be seen that the 

 whole depends upon a very simple cause, requiring neither the sub- 

 mergence nor the elevation of continents, nor any other great change 

 in the physical geography of the globe. 



When the excentricity of the earth's orbit is at a high value and 

 the Northern winter solstice is in perihelion, agencies are brought into 

 operation which make the S.E. trade winds stronger than the N.E., and 

 compel them to blow over upon the Northern hemisphere as far 

 probably as the Tropic of Cancer. The result is that all the great 

 equatorial currents of the ocean are impelled into the Northern 

 hemisphere, which thus, in consequence of the immense accumulation 

 of warm water, has its temperature raised, and snow and ice to a 

 great extent must then disappear from the Arctic regions. When the 

 precession of the equinoxes brings round the winter solstice to 

 aphelion, the condition of things on the two hemispheres is reversed, 

 and the N.E. trades then blow over upon the southern hemisj^here, 

 carrying the great equatorial currents along with them. The warm 

 water being thus wholly withdrawn from the Northern hemisphere, 

 its temperature sinks enormously, and snow and ice begin to ac- 

 cumulate in temperate regions. The amount of precipitation in the 

 form of snow in temperate regions is at the same time enormously 

 increased by the excess of the evaporation in low latitudes resulting 

 from the nearness of the sun in perihelion during summer. 



The final result to which we are, therefore, led, is, that those warm 

 and cold periods, which have alternately prevailed during past ages, 



