GEOLOGY 133 



rocks carrying the oldest known evidences of organic life 

 carry also indisputable evidence of extensive glaciation. In 

 fact, there is some evidence that even older rocks than these 

 show evidence of glacial periods — rocks that in age are re- 

 lated to the oldest fossil bearing rocks much as these latter 

 are related to the most recent oceanic sediments. 



In exactly a similar way does the geological evidence of 

 arid conditions during the past history of the earth fail to 

 coincide with what should occur under the nebular hypothesis. 

 Desert conditions have occurred at many times during the 

 past. In fact, probably the most widespread conditions of 

 aridity that have ever occurred in the earth's history occurred 

 in Permian and Triassic times — half way down the legible 

 geological record. At that time such conditions seem to have 

 been much more widespread than at present or at any time 

 since. 



According to the planetesimal hypothesis, the atmosphere 

 has developed gradually. The early nebular nucleus from 

 which the earth developed had no atmosphere. Gradually 

 the growing earth was able to hold nebular gases that came 

 within its reach from the outside and to give off gases from 

 its own interior that had been accumulated along with the 

 accretions of meteoric matter. The atmosphere, therefore, 

 began as an extremely attenuated gas and increased in density 

 either up to the present time or up to a point of equilibrium 

 or Balance between accumulation and loss and has remained in 

 equilibrium up to the present time. It is, therefore, probably 

 as dense now as it ever was. The doctrine of uniformitarian- 

 ism may, therefore, be applied to the development of the earth 

 from the very beginning of its astronomical development, and 

 it may be relieved of its period of glowing fiery heat when 

 conditions were as unlike those now existing as could well be 

 imagined. 



The nebular hypothesis gets the geologist into difficulty 

 again when he takes up the study of the beginnings of organic 



