252 POPULAR SCIENCE MONTHLY. 



ameliorating influences, as well as to life. Before it could have attained 

 its later phases, the acute thermal transparency of the atmosphere must 

 have given place to moderate absorption, and temperate conditions must 

 have succeeded cold. From waters warmed on widening shallows, car- 

 bon dioxide would pass into the air by simple diffusion and by chemical 

 dissociation. But the principal contribution, upon which generally pre- 

 vailing mildness would attend, would be associated with the active 

 development of lime-secreting life, and this relation is firmly estab- 

 lished by observation. 



Grand seasons of the eras are thus interpreted by Chamberlin as 

 effects of periodic adjustments of the earth's superficial form to stresses 

 developed within its mass. The causes of these stresses are sought by 

 physicists and geologists in the most profound researches, and for the 

 present, at least, they elude discovery, because the physical and chemical 

 conditions of matter within the earth transcend conditions of observa- 

 tion. But geologic investigation is competent to trace their influence 

 upon aspects of the earth, and not the least valuable result of Cham- 

 berlin's thought is the impulse it imparts to studies into the geography 

 and life of the past. 



The general hypothesis being thus promisingly developed, some 

 would have been satisfied there to rest the suggestion, and the general 

 reader may be content with the splendidly panoramic view of effects and 

 causes which it embraces. But its author pursues its analysis and appli- 

 cation with rigorous questioning, limited only by the bounds of existing 

 knowledge, and where knowledge fails he points out the need of 

 research. We shall touch only upon the principal points of his thorough 

 discussion, the competency of the causes, the oscillations of glaciation, 

 the time limits set by the probable duration of glacial and interglacial 

 epochs and the localization of glaciation in Pleistocene and in Car- 

 boniferous times. 



As already stated, the Pleistocene glaciation is attributed to deple- 

 tion of the atmospheric carbon dioxide occasioned by the notable expan- 

 sion and elevation of lands late in the Pliocene period. It is estimated 

 that in the preceding warm age the land area was 44,000,000 square 

 miles. That of the succeeding expansion at its maximum is computed 

 at 65,000,000 square miles, and the present extent is taken at 54,000,- 

 000 square miles. That is to say, the areas are related nearly as 

 1 :1% :li/4- Elevation, which is more important than extent, was at the 

 time of greatest expansion at least two or three times what it shortly 

 before had been when continents were smaller. In the earlier time of 

 mildness the margins of continents were generally submerged, as the 

 eastern portion of North America now is, affording a roomy habitat for 

 lime-secreting marine life. But with the uplift of continents these sea- 

 shelves were reduced to narrow zones along the steeps which descend 



