OENERAL LAWS OF ORIGIN AND MIGRATION. 253 



Maga- 



hcmispherc as far as the Tropic of Cancer. The result is 

 that all the <i;rcnt e(jiiatorial currents of the ocean are im- 

 pelled into the north tn hemisphere, wliich thus, in con- 

 sequence of thj immense 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 be- 

 comes clad in ice, and the southern has its temperature 

 raised. 



At the same time, according to CroH's calculations, 

 the accumulation 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 submergence of land would coincide with a cold 

 condition, and emergence with increasing warmth. Facts 

 already referred to, however, show that this has not al- 

 ways been the case, but that in many cases submergence 

 was accompanied with the influx of warm equatorial 

 waters and a raised temperature, this apparently depend- 

 ing on the (juestion of local distribution of land and 

 water ; and this in its turn being regulated not always by 

 mere shiftingof 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 ex- 

 cess of material abstracted by ice and frost from the arctic 

 regions, and drifted southward along the lines of arctic 

 currents. This drifting must in all geological times have 

 greatly exceeded, as it certainly does at present, the de- 

 nudation caused by atmospheric action at the equator, 

 and must have tended to increase the dispositioii to equa- 

 torial collapse occasioned by retardation of rotation.* 



While such considerations as those above referred to 



* Croll, in "C'liriiato ani3 Time," and in a note roarl before the Briti-'h 

 AB.sociali()n in 187»), takr-s an opposite view; but tliis in ch-arly t-ontrary 

 to the facts of hodiiiieiitaliitn, which show a steady movcuient of debrvi 

 toward the ijoutli and tjouthwest. 



