386 CAUSES OF CLIMATAL CHANGE 



respectively those of CroU and Lyell, or the astronomical and 

 geographical theories. 



The late Mr. Croll has, in his valuable work " Climate and 

 Time," and in various memoirs, brought forward an ingenious 

 astronomical theory to account for changes of climate. This- 

 theory, as stated by himself, is that when the eccentricity 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 south-east trade winds stronger than the north- 

 east, and compel them to blow over upon the northern hemi- 

 sphere as far 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 im- 

 mense accumulation of warm water, has its temperature raised, 

 so that ice and snow must, to a great extent, disappear from the 

 Arctic regions. In the prevalence of the converse conditions 

 the Arctic zone becomes clad in ice, and the southern has its 

 temperature raised. 



At the same time, according to Croll's calculations, the ac- 

 cumulation of ice on either pole would tend, by shifting the 

 earth's centre of gravity, to raise the level of the ocean and 

 submerge the land on the colder hemisphere. Thus a sub- 

 mergence of land would coincide with a cold condition, and 

 emergence with increasing warmth. Facts already referred to, 

 however, show that this has not always been the case, but that 

 in many cases submergence was accompanied with the influx 

 of warm equatorial waters and a raised temperature, this ap- 

 parently depending on the question of local distribution of 

 land and water ; and this, in its turn, being regulated not always 

 by mere shifting of the centre of gravity, but by foldings occa- 

 sioned by contraction, by equatorial subsidences resulting from 

 the retardation of the earth's rotation, and by the excess of 

 material abstracted by ice and frost from the Arctic regions, and 

 drifted southward along the lines of arctic currents. This drift- 



