Cause of Glacial Epochs. 377 



what is called the crust of the earth, but by no means 

 by sudden upheavals of vast mountain tracts at or near 

 the poles, or any where' else on the earth's surface ; and, 

 indeed, the phenomena of the vegetation of old geolo- 

 gical epochs in formations as far north as land has been 

 discovered, seems to me to point in that direction and 

 in no other. At all events it is plain, that no such 

 sweeping changes of physical geography have taken 

 place in those comparatively short episodes of geological 

 history, that have graduated into each other from the 

 beginning of this latest glacial epoch down to the 

 present day, and therefore it is needless to discuss the 

 question here. 



There is, however, an astronomical cause which 

 seems to meet all the circumstances of any one glacial 

 epoch, and is therefore deserving of the gravest 

 attention. The question has been worked out with 

 great skill by Dr. James Croll, first, in various memoirs, 

 and latterly, in his celebrated work ' Climate and Time,' 

 and I can only state in a very sketchy manner some of 

 his main conclusions. 



Alternations of cold and warm or temperate climates, 

 in the same latitudes, are in the first instance due to 

 the varying eccentricity of the orbit of the earth, by 

 which ' a host of physical agencies are brought into 

 operation, the combined eifect of which is to lower to a 

 very great extent the temperature of the hemisphere 

 whose winters occur in aphelion, and to raise to 

 nearly as great an extent the temperature of the 

 opposite hemisphere, whose winters of course occur 

 in perihelion.' It is perhaps possible that the orbit 

 of the Earth may become circular, at periods of time 

 prodigiously far removed from each other, but at 

 present, when the earth in its elliptical orbit is 



