ORIGIN AND DIFFERENTIATION 545 



of the earth's crust, since otherwise the earth would be heating up with geo- 

 logical rapidity, instead of being a body slowly cooling or in thermal equilib- 

 rium." 



It is not known to what depth in the earth the various radioactive 

 minerals extend, nor is it known what effect great pressure may have on 

 radio emanations; but inasmuch as these elements have the highest 

 atomic weights known, it would seem a fair presumption that there is a 

 progressively greater condensation of them toward the center. In any 

 event, it is obvious that the restriction of these radioactive elements 

 to the outer 40 miles of the earth's crust is a mere convention, devised 

 for the sole purpose of getting around what might otherwise be an awk- 

 ward condition of being obliged to account for too much heat. It would 

 seem to me that it is within the bounds of possibility that this may be 

 the key to an explanation of an augmented earth heat during the earlier 

 geologic ages. As it seems now to have been demonstrated with more 

 than a reasonable degree of certainty that the breaking down of the 

 radioactive elements into a descendant series, with the consequent evolu- 

 tion of heat, is a process that requires a measurable, albeit a very long, 

 period of time, it may be that this is the explanation of a higher earth 

 heat during the earlier ages, and that it reached its maximum long 

 prior to the Pleistocene, since which time it has been declining. This 

 offers an explanation of the present cooling globe or one in approximate 

 thermal equilibrium. But this is pure speculation, as difficult to prove 

 as to disprove. However, in the next paragraphs certain facts will be 

 set forth that seem to call for a higher inherent earth heat than we have 

 had since the beginning of the Pleistocene. 



OCEAN TEMPERATURES 



A word may be said at this point in regard to the temperature of the 

 ocean waters during early geologic time. It now seems to be settled 

 beyond serious question that the waters of the early oceans were warm- — 

 in fact, that they were not permanently cooled as they are now until the 

 approach of Pleistocene time. This does not necessarily mean that there 

 may not have been some fluctuation in their temperature from time to 

 time, for there doubtless was; but, taken by and large, the oceans were 

 warm from the equator to the poles. On this point Ulrich ^^ says : 



"Taking the geological marine record, as preserved in the fossiliferous rocks 

 from the Cambrian to the Tertiary, it suggests equable, mild, almost subtrop- 

 ical climates over the whole Northern Hemisphere in all the ages represented." 



20 E. O. Ulrich : Revision of the Paleozoic systems. Bull. Geol. Soc. Am,, vol. 22, 1911 

 p. 352. 



