544 F. H. KXOWLTOX EVOLUTIOX OF GEOLOGIC CLIMATES 



later became the arid belts) ; and thus gradually the zonal solar regime was 

 established." 



These are the essential features of Manson's theory and, whatever its 

 fate may be, I thoroughly agree with Hilgard that it must be given very 

 serious consideration. . Of course, objection has been raised against the 

 acceptance of this hypothesis of a higher initial and subsequent diminu- 

 tion of surface temperatures. We are told, for instance, that, owing to 

 the law of conductivity of the rocks, the transmission of earth heat to the 

 surface must have ceased as a practical factor very soon after the forma- 

 tion of an earth crust. We know, or think we know, that there is ample 

 heat in the earth^s interior, for if the ratio holds good of an increase of 

 one degree for about 60 feet of depth, we shall have the stupendous 

 temperature of 348,000° F. at the center. But conduction is not the 

 only means by which earth heat may be brought to the surface. Faults, 

 fractures, denudation, thermal waters, orogenesis, and, of course, vulcan- 

 ism are continually liberating appreciable amounts of heat. On the 

 basis of vulcanism — if this is a really competent criterion — the earth 

 appears to have been cooling since earlier geologic time; for, though by 

 no means extinguished, vulcanic activity is much less evident now than 

 it has obviously been in the past. 



Radioactive heat. — But, fortunately or otherwise, we are not depend- 

 ent for a possible solution of this problem on an original earth heat, 

 whether developed from a cooling nebula or by impact and pressure of 

 separate solid increments. The recent wonderful discovery of radio- 

 activity introduces a wholly new and unsuspected set of factors; but 

 the facts are so newly available that the possibilities and limitations are 

 not yet fully worked out. It is now known that uranium and thorium, 

 elements having the highest atomic weights known, are the parents of a 

 series of radioactive elements. Each of these elements breaks down 

 into a descendant series by giving off positively charged atoms of helium 

 at an enormous velocity, by negatively charged electrons, also with high 

 velocit}^, and by certain rays of the nature of Eontgen or X-rays. After 

 a very long but measurable length of time a stable element is the result, 

 as, for instance, lead, which results from the breaking down of uranium. 

 This process is necessarily accompanied by the evolution of heat ; in fact, 

 as Barrell ^^ has said : 



"It has been found that radioactivity gives such an embarrassingly large 

 quantity of heat that it has been necessary to assume the restriction of 

 uranium and thorium with their observed percentages to the outer -iO miles 



2s Joseph Barrel! : Rhythms and measurements of geologic time. Bull. Geol. Soc. Am. 

 vol. 28, 1917, p. 839. 



