HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 567 
Cretaceous period, and this fact may be taken as indicating that 
the planation which reached so pronounced an expression on 
the American continent at that period affected nearly or quite 
all the continents in some similar measure. An inspection of 
the whole range of geological history shows periods of similar 
transgression accompanied by the development of luxurious and 
cosmopolitan marine faunas of the shallow-water type which in 
themselves imply the conditions here postulated.’ 
If this be a correct view it is obvious that at such periods 
the areas of land exposed to atmospheric action were notably 
reduced by sea encroachment, and that at the same time the 
lowness of the land greatly limited the depth of atmospheric 
activity by reducing the zone between the surface and the water- 
table and by reducing the hydrostatic penetration of surface 
waters. As already remarked, there is to be counted in offset 
probably warmer temperature, a higher degree of moisture, and 
a more abundant vegetation, but it is not believed that this 
approaches, even remotely, to a full offset to the reduction due 
to low elevation and reduced area. 
If the foregoing views are correct there were certain periods 
in the history of the earth when carbonation proceeded with 
multiplied activity, separated by other periods during which its 
activity was greatly reduced. The intensification and the reduc- 
tion differ by some notable multiplier of the average rate. 
In working application, the hypothesis tentatively recognizes 
as periods of land extension attended by rapid carbon dioxide 
consumption, (1) the close of the Silurian and the opening of 
the Devonian, (2) the Permian and early Triassic, and (3) the 
Pliocene and Pleistocene. To this category may perhaps also 
belong, though the evidence at present is less adequate, (4) the 
early Cambrian, (5) the closing Ordovician and opening Silurian, 
*For further statement of these views, see The Ulterior Basis of Time Divisions 
and the Classification of Geologic History, Jour. GEOL., Vol. VI, No. 5, July— 
August, 1898, pp. 449-462; A Systematic Source of Evolution of Provincial Faunas, 
Jour. GEOL., Vol. VI, No. 6, Sept.—Oct., 1898, pp. 597-608; The Influence of Great 
Epochs of Limestone Formation upon the Constitution of the Atmosphere, zé77., pp. 
609-621. 
