248 CARNEGIE INvSTITUTION OF WASHINGTON. 



assigned the earth at a similar size, /. t'., composed largel)^ of the 

 heat-absorbing carbon dioxide. 



Without attempting to fix the precise stage, it is not unreasonable 

 to assume that surface waters had begun their accumulation upon 

 the earth's exterior wdiile 3^et it lay 1,500 to 1,800 miles below the 

 present surface. The present difference between the radii of the 

 oceanic basins and the radii of the continental platforms is scarcely 

 3 miles, on the average ; so that if the continental segments be 

 assumed to be in approximate hydrostatic equilibrium with the 

 oceanic segments toda}^, as seems highly probable, the selective 

 weathering process brought about a difference in depression of only 

 I mile in 500 or 600 miles, or about one-fifth of i per cent. 

 We appear, therefore, to be laying no heavier burden upon weather- 

 ing than it is competent to bear. It might well be thought to do 

 much more, but the process of w^eathering is slow, and as new ma- 

 terial was constantly falling in and burying the old, partial altera- 

 tion was all that could take place ; and, besides, a part of the basic 

 material leached from the surface was redeposited beneath the 

 ground water of the land and in landlocked basins and was not lost 

 to the continental segments. 



Not onl}^ is the evolution of the great ab^^smal basins and of the 

 continental platforms thus assigned to a very simple and inevitable 

 process, but there is therein laid the foundation for subsequent de- 

 formation of the abysmal and continental type. 



There is no direct evidence as to the time or the method of the 

 introduction of life upon the earth. The earliest legible record of 

 life in the form of fossils bears evidences of great advances in evolu- 

 tion along many divergent lines. The inference is therefore im- 

 perative that the initial forms of life had been introduced long 

 before, or else that an evolution quite out of harmony with that 

 which succeeded took place in the unknown inten^al antecedent to 

 the record. Whence the life was introduced is also quite unknown. 

 The speculation that it might have been brought to the earth from 

 some other celestial body by a fragment in the form of a meteorite 

 is merely a refuge from supposed geological, biological, and philo- 

 sophical difficulties — a merely temporar}' refuge in the face of pro- 

 digious improbabilities, for it only throws back the problem of life 

 genesis without solving it. There is nothing in known meteorites 

 save, perhaps, the existence of hydrocarbons equally assignable to 

 inorganic sources, to indicate that they came from worlds with at- 

 mospheres and hydrospheres suited to maintain such life as the 



