SUPPOSED CAUSES OF. 245 



and will yet occur, in the history of our globe. Of course, 

 before such hypotheses can be entertained, astronomers 

 must admit their possibility, and find in the existing 

 ordainings of nature some proofs that such great periodic 

 changes are still in process of fulfilment. It is true that 

 we are not without evidence of colder and warmer periods 

 over the same latitudes in the geological history of our 

 planet. It is many years ago * since we first drew attention 

 to the occurrence of colder and warmer cycles the Cam- 

 brian, Old Red, Permian, Cretaceous, and Boulder- Drift, 

 being the cold, and the Silurian, Carboniferous, Oolitic, 

 Tertiary, and Recent, being the warm ; but we still need 

 further corroboration of this, and it were the wisdom of 

 geology to exhaust the operative causes that lie within our 

 knowledge in the earth before seeking for extraneous causes 

 that are placed in the mean time beyond our probation. In 

 either case we cannot conceive of colder and warmer cycles 

 occurring at random ; and whether they depend upon the 

 earth's own ordainings, or upon her wider planetary re- 

 lations, they must be obedient to an orderly law of time, 

 and will occur in the future as they have already taken 

 place in the past. 



Of course, before the origin and history of the glacial 

 epoch can be fully expounded, much exact and prolonged 

 labour must be devoted to the subject. The sequence and 

 superposition of the clays, gravels, boulders, and silts, re- 

 quire more thorough examination over wider areas than 

 those of the British Islands. The southerly limits of these 

 accumulations in Europe, Asia, and America require to be 

 determined with greater precision ; as well as the question 

 of their altitude, and their dispersion over eastern and 

 central Asia. We have also to learn how far the southern 

 hemisphere (in Patagonia, Australia, and New Zealand) has 

 * See ' The Past and Present Life of the Globe/ p. 189-191. 



