THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 245 



be poised on a ticklish combination of conditions, and it nia}', indeed, 

 prove, when critically studied, to ])e reall}^ so, liut 3'et it is suliniitted 

 that it follows along- coherent lines connected ultimately with the fun- 

 damental proposition that dispersed meteoroidal matter might gather 

 in slowl}" rather than precipitately. On this point hangs all the law 

 and the prophets. 



If astronomers, physicists, and mathematicians will jointly attack 

 the forraational history of the solar system stage by stag-e, following 

 each stage out into details of time and rate, and taking* full cognizance 

 of all the alternativ^es that arise at each stage, it will then be possible, 

 perhaps, to decide whether the conditions of the early earth were such 

 as to require a large or a small amount of heat from the sun for the 

 sustenance of life, and whether the sun was wasting heat prodigally in 

 those days or conserving it for later expenditure. The present meas- 

 ure of the earth's needs may be no measure of its early needs. The 

 sun's present expenditure may be no measure of its early expenditure. 



In view of all these considerations, I again beg to inquire whether 

 there is at present a solid basis for any "sure assumption" with refer- 

 ence to the earth's early thermal conditions, either internal or external, 

 of such a determinate nature as to place any strict limitations upon 

 the duration of life. 



The latter part of the address is concerned with novel suggestions 

 regarding the behavior of the supposed liquid surface of the earth in 

 the stages just preceding its final solidification, involving a theory of 

 the formation of the primitive surface rocks and of the original con- 

 tinents and ocean basins. The discussion of this I must leave to the 

 petrologists, merely venturing the hint that they may find some occa- 

 sion to reconstruct current petrological doctrines if they are to be 

 brought into consonance with the new views offered. 



The point of greatest general interest in this part of the address is 

 the sharp statement of opinion that, if the original lava ocean had 

 solidified equably in all its parts and produced a dead-lev^el surface all 

 around the globe, there seems no possibility that our present conti- 

 nents could have arisen to their present heights, or the ocean basins 

 have supk to their present depths, during twenty or twenty-five million 

 years, or during any time, however long. (Exact words previously 

 quoted.) Lord Kelvin adds: 



Rejecting the extremely improbable hypothesis that thp continents 

 were built up of meteoric matter tossed from without upon the already 

 solidified earth, we have no other possil)le alternative than that they 

 are due to heterogeneousness in different parts of the liquid whic 

 constituted the earth before its solidification. 



This is as strong an assertion of the necessit}' of assuming crusta' 

 and subcrustal heterogenity^as any advocate of a slow-accretion earth 

 could wish. If the word "liquid" and what follows be stricken out, 



