AND ITS INHABITANTS 51 



recently, geologically speaking, was the present thin condition 

 of the atmosphere brought about. 



Even though more than sixty years ago a few English 

 geologists who were familiar with the geology of India and 

 Africa began to point out that the earth had passed through 

 a cold period toward the close of the Paleozoic era (see Figs. 

 4 and 5 ) , this discovery did not at once break down the teach- 

 irflPof Laplace and Dana. When, however, still older glacial 

 periods began to be discerned in the stratified rocks and espe- 

 cially in the oldest ones near the beginning of the Proterozoic 

 era (see Figs. 7 and 8) a record that is remarkably well 

 preserved in Canada the final straw was at hand to break 

 down the theory that the climates of the earth had passed 

 progressively from an astral through a torrid to a temperate 

 condition, along with a slow clarifying of the atmosphere (see 

 Figs. 3-8). 



The newer geology holds, with Chamberlin, that the grow- ' 

 ing earth was originally too small in mass to hold an atmos- 

 phere and accordingly there was a time when our planet had 

 none. As it grew in diameter, the earth was more and more 

 able to hold an atmosphere and a hydrosphere, and during 

 the long growing period there was finally originated an atmos- 

 phere probably not much unlike the present one, which in a 

 roughly approximate way may be said to be composed of 

 four-fifths nitrogen and one-fifth oxygen. It should, however, 

 be noted here that at first the atmosphere had but little oxygen. 

 Since life came to be, using the carbon and freeing most of 

 the oxygen of the carbon dioxide of the atmosphere, the 

 quantity of oxygen has more or less steadily increased, though 

 at all times much of it has been consumed in the oxidation 

 processes of the rocks and minerals. 



The most constant accessory constituent of air is carbon 

 dioxide, one of the three fundamental materials at the basis 

 of life. In the present atmosphere there are about three vol- 



