Decembeb 25, 1908] 



SCIENCE 



899 



atmosphere radically greater than the 

 present one, it is obvious that the notion of 

 an atmospheric pressure appreciably 

 greater than the present should not be 

 entertained. It is because this doctrine of 

 Stoney is believed to be fundamentally 

 sound, subject to some qualifications, that 

 the planetesimal hypothesis has developed 

 its postulates in consonance with it and has 

 not consciously entertained assumptions 

 regarding sequential states that are at 

 variance with it. If the doctrine of 

 Stoney is sound, the gaseous and gaseo- 

 meteoritic hypotheses have no advantage 

 as to the pressures of the early atmos- 

 pheres; on the contrary, the higher tem- 

 peratures they assume imply greater molec- 

 ular activity and hence greater molecular 

 losses and less pressures than prevail 

 around the present relatively cool earth. 

 In this paper, therefore, no appeal wiU be 

 made to general atmospheric pressures ap- 

 preciably greater than the present ; indeed, 

 we shall try to be true to the assumption 

 that the general atmospheric pressures in 

 the formative stages of the earth were 

 lower rather than higher than they are at 

 present. "What volume of atmosphere and 

 what degree of atmospheric pressure may 

 have been compatible with the gravitative 

 power of the earth at such early stages as 

 were first suitable for the evolution and 

 preservation of organic compounds is not 

 yet definitely postulated by the planetes- 

 imal hypothesis. It is merely assumed 

 that a suitable atmosphere, hydrosphere, 

 temperature and other conditions were 

 found at some one of the progressive 

 stages leading up to the present state and 

 that there was a gradual transition from 

 that stage to the existing state of things. 

 So far as gravity alone is concerned, a 

 sufficient atmosphere and hydrosphere 

 might apparently be held when the earth 

 had about one tenth its present mass, i. e., 

 about the present mass of Mars, which ap- 



parently holds an atmosphere and perhaps 

 a hydrosphere. 



If the earth were formed by a gathering 

 of planetesimals into a nucleus which at an 

 earlier stage was one of the nebulous knots 

 of a spiral nebula, the value of the mass 

 at the time it first became a solid body 

 should have hung upon the ratio of the 

 original knot to the scattered nebulous 

 matter destined to be gathered into it, con- 

 cerning which there is little to guide 

 opinion. In this uncertainty, let it be as- 

 sumed that at the stage when the growing 

 earth was ready for the critical synthesis, 

 its mass lay somewhere between one tenth 

 and three fourths of the present mass, 

 and that the mass of the early atmosphere 

 held a proportionate ratio to the mass of 

 the existing atmosphere. Let it also be as- 

 sumed that at the time at which the hydro- 

 sphere began to gather about the globe, the 

 earth's surface was formed of a hetero- 

 geneous, uneven, talus-like mantle, such as 

 the infalling planetesimals would natural- 

 ly produce, and that this constituted a 

 deep porous zone. Until the hydrosphere 

 was added there was a lack of efficient 

 solutions and inwash to cement the frag- 

 mental material. The hydrosphere, in its 

 first stages, must have occupied the lower 

 part of this porous zone, and must have 

 crept upwards as its volume increased. It 

 should have appeared at the surface first 

 in the bottoms of the deformation sags, the 

 innumerable craters and the other depres- 

 sions, and as it gradually extended itself, 

 it must have linked more and more of the 

 isolated water-bodies together. Through- 

 out the earlier stages of growth, the pre- 

 vailing aspect should have been that of in- 

 numerable small water-bodies scattered 

 over the face of the land, rather than that 

 of the great confluent ocean of to-day. 



This then is the general physiographic 

 setting which the planetesimal hypothesis, 

 at least in one of its phases, presents for 



