METHODS GEOLOGICAL 25 



land-areas of the northern hemisphere were affected, either 

 simultaneously or in rapid succession. 



Until a comparatively short time ago, it was very generally 

 believed that the Glacial or Pleistocene epoch, which was so 

 remarkable and conspicuous a feature of the Quaternary period, 

 was an isolated phenomenon, unique in the entire history of 

 the earth. Now, however, it has been conclusively shown 

 that such epochs of cold have been recurrent and that no less 

 than five of these have left unmistakable records in as many 

 widely separated periods of time. 



When the hypothesis of a great ' ' Ice Age " in the Pleistocene 

 was first propounded by the elder Agassiz, it was naturally 

 received with general incredulity, but the gradual accumula- 

 tion of proofs has resulted in such an overwhelming weight 

 of testimony, that the glacial hypothesis is now accepted as one 

 of the commonplaces of Geology. The proofs consist chiefly 

 in the characteristic glacial accumulations, moraines and drift- 

 sheets, which cover such enormous areas in Europe and North 

 America and, on a much smaller scale, in Patagonia, and in the 

 equally characteristic marks of glacial wear left upon the 

 rocks over which the ice-sheets moved. Many years later 

 it was proved that the Permian period had been a time of 

 gigantic glaciation, chiefly in the southern hemisphere, when 

 vast ice-caps moved slowly over parts of South America, 

 South Africa, Australia and even of India. The evidence is 

 of precisely the same nature as in the case of the Pleistocene 

 glaciation. In not less than three more ancient periods, 

 the Devonian, Cambrian, and Algonkian, proofs of glacial 

 action have been obtained. 



While the rocks themselves thus afford valuable testimony 

 as to the climatic conditions which prevailed at the time and 

 place of their formation, this testimony is fragmentary, missing 

 for very long periods, and must be supplemented from the 

 information presented by the fossils. As in all matters where 

 fossils are involved, the evidence must be cautiously used, for 



