AND ITS INHABITANTS 57 



of the calculations arrive at about 100 million years. This 

 figure is based on the postulate that the present rate has been 

 continuous throughout geologic time; this rate, however, 

 appears to be excessive, and renders the postulate erroneous. 

 The rate was highly variable during geologic time, as will be 

 seen from the evidence throughout this chapter, so that the 

 latest calculation of Holmes 3 gives the time as between 210 

 and 340 million years, with the warning that these high figures 

 "must not be supposed to possess any serious value. . . . 

 For the present we can only conclude that our knowledge of 

 the part played by sodium and chlorine in the constant redis- 

 tribution of the materials of the earth's crust is still lamentably 

 imperfect, and that quantitative deductions drawn from it 

 must be regarded as being purely provisional." 



Origin of the sedimentary strata. The great variability of 

 geologic climates also leads to variations in the power of the 

 atmosphere to weather and break down the high places of 

 the lands. The earliest atmosphere, far richer in carbon 

 monoxide and carbon dioxide than subsequent climates, was 

 a most active weathering agent in dissolving and breaking up 

 the oldest rocks of the lithosphere, the igneous basaltic and 

 granitic rocks. Then, too, there had not yet developed a 

 vegetation on the most ancient lands to hold the fast-forming 

 soils, and' these conditions made it inevitable that all of this 

 loose and solvent material should be rapidly washed by the 

 rains and rivers into the oceanic basins. Therefore the strati- 

 fied or water-laid rocks accumulated during the earlier geologic 

 eras more quickly than in the later ones. On the other hand, 

 the erosive power of both air and water has varied much 

 throughout geologic time because of the periodically varying 

 height of the land and the ever changing humidity and tem- 

 perature of the air. Accordingly, the rivers carry their loads 

 of sediments for long times very slowly and at others much 



3 Holmes, A., op. cit., pp. 74-75. 



