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
1⁄2
:1
1⁄4
- 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
