AND ITS INHABITANTS 63 



that over the remaining small portion they vary from i to 

 20 miles; yet the greater depths do not cover more than one- 

 eighth of the continent, and they occur in the long and narrow 

 troughs of sedimentation, of which the two best known lie in 

 the Appalachian and Rocky Mountain regions (see Fig. 9). 



Eras of geological time. Now let us see how geologists 

 divide this great pile of at least 53 miles of sediments and 

 their included fossils. (A synopsis of geologic time is pre- 

 sented on page 46, and in Fig. 3.) The first era, or Archeo- 

 zoic time, with the oldest known strata, has no proved fossils, 

 though a very low form of water-living alga appears to have 

 been present. Even if we exclude the probable fossil evidence, 

 the presence in Ontario of 18 miles of sediments, of which 

 about one-half is impure limestone along with much graphite, 

 indicates unmistakably that life was already in existence. As 

 we are treating of the oldest known time in the history of the 

 earth, this is the place to emphasize what results were then 

 attained and what natural processes were at work. 



At the very beginning of the Archeozoic era, the earth had 

 a rocky and fairly stable exterior known as the lithosphere, an 

 atmosphere and a hydrosphere; protruding continents, and 

 oceanic basins that were filled with slightly saline water; twice- 

 daily tides, wear and tear of the ocean waves against the 

 lands, reduction of the high lands by the atmosphere, and, 

 through the rains, washing of the soil and solution materials 

 into the seas and oceans. The sun shone then as now and 

 made life possible, and the man in the moon was as clearly 

 developed as he is today, remaining so ever since because this 

 satellite has had neither atmosphere nor water to wash away 

 his face. We therefore see that the earth was then very much 

 as it is now, with these great differences, that at first the 

 atmosphere was almost devoid of free oxygen and probably 

 richer in carbon dioxide, that the lands were not covered with 

 verdure, and that what there was of life was of a very low 



