ORIGIX AND DIFFERENTIATION 589 



most, not been continuous and uninterrupted, for on no other basis do 

 certain observed facts seem explainable. For example, polar geniality of 

 climate, such as occurred again and again in the ages before the Pleisto- 

 cene, can not be explained on the basis of unmodified solar control. Gla- 

 ciation such as that of the "Permo-Carboniferous,'' which occurred in 

 and adjacent to the tropics, is impossible under a vertical or nearly verti- 

 cal sun, or at least not Avithout an elevation of the land-mass that is out 

 of all proportion to the observed stratigraphic and structural data. 



In other words, it seems evident that there must have been some other 

 factor or factors that operated to obscure or modify solar control. Can 

 these be predicated with any reasonable degree of certainty? 



COXSIDERATIOX OF THE ATMOSPHERE 



In seeking a possible answer we may first consider the atmosphere. It 

 need hardly be recalled that the atmosphere, the outer gaseous envelope 

 surrounding the earth, is a mechanical mixture of oxygen, nitrogen, a 

 small increment of carbon dioxide, and still smaller quantities of various 

 other gases, together with water vapor in amounts that vary with tem- 

 perature, locality, and season. The height to which the atmosphere ex- 

 tends above the earth is not known, though it exceeds 100 miles, and one- 

 half the air is below the 20,000-foot level. According to Humphreys,-^ 

 "At an elevation that in middle latitude averages about 11 kilometers 

 [36,000 feet] the temperature of the atmosphere becomes substantiallv 

 constant, or, in general, ceases appreciably to decrease with increase of 

 elevation; this is, therefore, the upper limit of distinct vertical convec- 

 tion and of cloud formation." 



A beam of sunlight in passing from the earth to the sun has the rapidly 

 vibrating light waves transformed in large part at the surface of the 

 earth into slower vibrating heat waves. In passing from the outer limit 

 of the atmosphere to the surface of the earth, the beam of sunlight is 

 subject to certain losses of energy, such as absorption by the gases and 

 vapors of the atmosphere, especially the water vapor bands, the "scattering 

 of light toward the ground from the direct solar beam by the molecules 

 and dust particles," and from the loss by scattering into space by these 

 dust particles. Now as to the effect of the atmosphere itself : It is well 

 known that the oxygen and nitrogen are transparent to both incoming 

 light rays and outgoing heat rays, Avhile the water vapor and carbon 

 dioxide are largely opaque to the outgoing heat rays. This, then, is the 

 crux of the whole matter. The water vapor in the atmosphere, aug- 



25 W. J. Humphreys : Volcanic dust as a factor in tlie pi-oduction of climatic change. 

 Washington Acad. Sci. .Jour., vol. 3, 191.3, p. 867. 



