W. TJpham — Diversity of the Glacial Drift. 359 



noinic period mentioned, giving seven or eight epochs of 

 glaciation and as many interglacial epochs when the ice-sheets 

 were melted away; but continuous temperate conditions simi- 

 lar to those of the present would have prevailed during the 

 past 50,000 years or more. The recency of the date marking 

 the close of the Ice age is inconsistent with the astronomic 

 theory. But under the fruitful incentive of that theory many 

 glacialists in Europe and America have interpreted their obser- 

 vations as establishing the recurrence of glacial and inter- 

 glacial epochs which it suggested ; and some who distrust or 

 reject astronomic causes for the Ice age continue to hold this 

 interpretation of the records of the glacial drift. 



Looking through the long past ages previous to the Pleisto- 

 cene, we come to no time affording evidences of widely ex- 

 tended glaciation, probably affecting continental areas, till we 

 pass back at least many million years. Only one earlier stage 

 of the earth's changes was attended, so far as geology can tell 

 us, with the envelopment of large land areas beneath ice- 

 sheets, and this was in the Permian period, closing the Paleo- 

 zoic era. It was a time of great orogenic and epeirogenic 

 changes ; and I think that then, as in the Pleistocene Ice age, 

 the accumulation of thick sheets of land ice was due to great 

 epeirogenic uplifts of those areas so high as to give them a 

 cool climate and chiefly snowfall instead of rainfall through- 

 out the year. If extensive glaciation has been so rare, shall 

 we readily believe that during the geologically very short 

 Quaternary or Psychozoic era there have been two or three or 

 several Glacial epochs? More probably, as I think, we shall 

 find all the diverse phases of our glacial drift referable to a 

 single and continuous Ice age ; and the very slight changes of 

 marine molluscan faunas during this age implies its excep- 

 tional brevity in comparison with any of the preceding periods 

 recognized by geologists. 



Earlier and Later Drift near its boundaries in the Missis- 

 sippi basin. — The admirable work of Profs. Chamberlin and 

 Salisbury on the Wisconsin driftless area and farther south in 

 the Mississippi basin discriminates earlier, mostly thin, and 

 later, thick and morainic, varieties of the drift border. These 

 differ widely in their volume of drift, in its constituent mate- 

 rial, and in the times of its deposition. Along the greater 

 part of the boundaries of the drift on those areas, it terminates 

 in an attenuated border, slowly thinning out and presenting 

 considerable difficulty for the recognition and mapping of its 

 limits. Indeed, it is apt to occur on its outermost tracts in 

 low and thin, smooth patches, more or less isolated, and these 

 are thought to represent in large degree the original method 

 of deposition, not being a result of subsequent erosion. These 



