4<o Sayles — Dilemma of Paleoclimatolo gists. 



unless we say that continental emergence cooled off the 

 earth a little, and that the major changes were due to great 

 changes in sun's heat; or that the sun changed gradually 

 through the Pliocene and then more abruptly to bring in 

 the glaciers, and then changed just as abruptly in the 

 reverse direction to bring on more heat again ? It is con- 

 ceivable that the sun has a regular period of change of 

 great length, of an oscillative nature, compared to which 

 the sunspot period would be the merest ripple on a great 

 wave. With the greater length of geological time now 

 becoming evident, such a change in the sun might very 

 well come during a time when the lands were highly 

 emergent and the earth cool. At other times when the 

 continents were submerged such changes in the sun could 

 not bring on great glaciation, but might cool the earth con- 

 siderably and produce local glaciation. As noted above, 

 Croll thought the barren strata indicated periods of cold. 

 The volcanic dust hypothesis advocated by Hum- 

 phreys 22 has an advantage over the solar hypothesis in 

 that we need not go beyond the earth itself for an explana- 

 tion of interglacial phases. Periods of vulcanism fol- 

 lowed by times of quiet clear air would explain glacial 

 and interglacial episodes. There have been many periods 

 of great vulcanism without accompanying glacial periods, 

 but in these cases it is most probable that the earth was 

 either too warm to be brought to a condition of glaciation, 

 or that vulcanism was too spasmodic in its action to have 

 a prolonged cooling effect, or it is possible that evidence 

 exists or existed for such cooled periods but that it 

 remains undiscovered or lost. It would not be remark- 

 able if such evidence were lost. Suppose, for example, 

 that we have at present reached the end of the Pleistocene 

 and are now at the beginning of a new warmer period of 

 earth history. Unless the glaciated areas sink rela- 

 tively soon there will be very little to preserve as evidence 

 of Pleistocene glaciations. The marine Pleistocene gla- 

 cial deposits are very limited. The chances that these 

 will be uplifted are very uncertain. Such thoughts as 

 these lead one to conclude either that the Pleistocene 

 glaciation was very limited or that there is much more to 

 come, and that we are now living in an interglacial time. 

 If a small remnant of till should be preserved, would the 

 Pleistocene be regarded as just a local glaciation? There 



