THE PLEISTOCENE OR GLACIAL PERIOD. 337 



region, 1 and consonant phenomena elsewhere, indicate only a limited 

 extension of the ice beyond its present border. The Arctic islands 

 west of Greenland seem, from present evidence, to have been only 

 partially glaciated, though the ice extended considerably beyond its 

 present limits. 



Summary. — Reviewing comprehensively the distribution of the 

 ice, it appears that by far the greatest Pleistocene glaciation was 

 developed in the northern hemisphere, and that its most significant 

 portion was the glaciation of the great lowland areas of northeastern 

 North America. This glaciation reached its climax of significance 

 in the deployment of the Keewatin ice-sheet from a low, flat center, 

 in seeming, but doubtless not real, negligence, or even defiance of 

 topographic relations, and to some extent of climatic conditions as well. 



The Criteria of Glaciation. 



So extraordinary a series of phenomena as the repeated burial 

 of half the plains of North America beneath sheets of ice which spread 

 southward into mild temperate latitudes, could not be accepted on 

 other than the most cogent evidence, and it is not strange that the 

 glacial theory was resisted for half a century, though the iceberg and 

 other glacio-natant hypotheses urged in its stead seem no more credi- 

 ble, and far less adequate. But the cumulative force of a vast mass 

 of evidence, rigorously scrutinized under the promptings of this criti- 

 cal and reluctant attitude, has become overwhelming, and the days 

 of reasonable doubt are passed. The decisive evidence lies not only 

 in a great mass of individual criteria, but in a combination of con- 

 vergent lines of proof which lend invincible support to one another. 



The area which was overspread by ice is covered by a mantle of 

 clay, sand, and bowlders, which, taken together, constitute the drift. 

 Some of the drift is stratified (Fig. 472), but more of it is without the 

 assortment and the definite arrangement which goes with stratification 

 (Fig. 473). The various lines of evidence which have led to the gen- 

 eral acceptance of the glacial theory, have to do with (1) the drift, 

 (2) the surface of the rock which underlies it, and (3) the relations 

 of the drift to its bed. Some of the principal considerations are the 

 following: 2 



^hamberlin, Jour, of Geol., Vol. I IT, 1895. 



2 The phenomena pointing to the glacial origin of the drift have become so fa- 



