662 HISTORICAL GEOLOGY 



has been favorably received, especially in America, Carbon di- 

 oxide and water vapor act much as does the glass in a greenhouse, 

 i.e., they form a thermal blanket which prevents the radiation of much 

 of the heat derived from the sun. When the amount of one or both 

 of these gases is diminished, the radiation increases, and the climate 

 becomes colder. During periods of great elevation and continental 

 extension, erosion would be greatly increased, and the withdrawal of 

 carbon dioxide from the air would be rapid. This would be the case 

 since carbon dioxide is consumed in large quantities by rocks in 

 weathering. Such consumption, under conditions favorable to great 

 erosion, may be in excess of the supply. Also, at times of great land 

 extension the water surfaces are relatively small, and since less evap- 

 oration occurs there is diminution of the water vapor in the air. 

 The elevation of the land at the close of the Tertiary favored the con- 

 sumption of carbon dioxide, and the contraction of the oceans fur- 

 nished less water vapor to the atmosphere than formerly. " By 

 variations in the consumption of carbon dioxide, especially in its 

 absorption and escape from the ocean, the hypothesis attempts to 

 explain the periodicity of glaciation. Localization is attributed to 

 the two great areas of permanent low pressure in proximity to which 

 the ice sheet developed." (Chamberlin and Salisbury.) 



REFERENCES FOR CAUSES OF GLACIATION 



Chamberlin, T. C, — An Attempt to Frame a Working Hypothesis of the Cause of 

 Glacial Periods on an Atmospheric Basis: Jour. Geol., Vol. 7, 1899, pp. 545-584; 

 667-685; 75I-787- 



Chamberlin and Salisbury, — Geology, Vol. 3, pp. 424-446. 



Clarke, F. W., — Data of Geochemistry : Bull. U. S. Geol. Surv. No. 616, pp. 47, 

 48, and 143-147. 



Croll, James, — Climate and Time and their Geological Relations. 



Dana, J. D., — Manual of Geology, p. 978. 



LeConte, J., — Elements of Geology, 5th ed., pp. 612-619. 



Effects of Glaciation 



Glaciation benefited some regions and was harmful to others, but 

 the former effects were, on the whole, greater than the latter. 

 Among other benefits may be mentioned (1) waterfalls and rapids, 

 which afford valuable water power; (2) lakes which not only afford 

 means of transportation, but so ameliorate the climate as to per- 

 mit the raising of fruits, such as peaches and grapes, which otherwi < 

 would not thrive. In addition to this, they beautify the rejri 



