240 



SCIENCE 



[N. S. Vol. XLIV. No. 1129 



long been desired by geologists as well as other 

 scientists. The method of treatment conforms 

 to that followed by the University of Chicago 

 Science Series. This requires that the sub- 

 ject shall be presented " in as summary a man- 

 ner and with as little technical detail as is 

 consistent with sound method. These volumes 

 will be written not only for the specialist, but 

 for the educated layman." 



The previous publications on the planetesi- 

 mal hypothesis and its relations to the origin 

 of the earth are found in articles chiefly by 

 Chamberlin in the Journal of Geology, chiefly 

 by Moulton in the Astro-Physical Journal 

 and by various collaborators, mostly in publi- 

 cations of the Carnegie Institution; but only 

 the specialist has pursued all of this more or 

 less technical literature to its lairs. In addi- 

 tion, Moulton has given some account of the 

 astronomic aspects of the hypothesis in his 

 text-book of astronomy and Chamberlin and 

 Salisbury in 1906 have given considerable 

 space to the subject in their " Geology," espe- 

 cially the first eighty pages of Volume II. 

 These are works which are not readily access- 

 ible to the educated layman. Furthermore, 

 the present volume by going straight to its end 

 and omitting technicalities brings the essen- 

 tial framework of the hypothesis into better 

 relief and perspective than is the case in 

 Chamberlin and Salisbury's " Geology." Pub- 

 lished ten years after the latter, it furthermore 

 takes advantage of the research of a later 

 decade. There is added also an essentially new 

 chapter on " The Juvenile Shaping of the 

 Earth." 



The form of presentation is very readable 

 and attractive. It follows largely the intellec- 

 tual trail which led the author, as he says, a 

 specialist in glacial geology, " across the pass 

 that leads from the land of rocks into the realm 

 of cosmogonic bogs and fens. Its mists were 

 already gathering over the path ahead. 

 Strangely enough, the cold trail of the ice 

 invasion had led by this long and devious path 

 into the nebulous field of genesis." 



All older views of the origin of the earth 

 had grown up around the idea that the matter 

 which constitutes the planets was a residuum 



left from the primal gathering in of the solar 

 nebula. This process had been given concrete 

 form in the nebular hypothesis of Laplace. 

 But an examination of the stubborn facts ex- 

 pressed in the structure and motions of the 

 solar system brought out dynamical inconsis- 

 tencies with the terms of the nebular hypoth- 

 esis. Modified forms of that hypothesis could 

 not overcome the objections. Therefore Cham- 

 berlin was led to build up an hypothesis of earth 

 origin from a totally different beginning. He 

 postulated an ancestral sun already condensed 

 and sought to derive the planetary matter and 

 planetary energies of motion from the expul- 

 sive forces set up by the close approach and 

 passage of another star. The result was the 

 development of a great swarm of larger and 

 smaller particles revolving independently but 

 nearly in one plane in elliptic orbits about the 

 sun. This is the basal foundation of the 

 planetesimal hypothesis. In these respects it 

 is in direct opposition to the Laplacian hypoth- 

 esis and in considerable opposition to the 

 meteoritic modifications from that hypothesis. 



The subject is vast and the evidence on 

 many aspects is somewhat vague. A variety 

 of subhypotheses could be raised for compari- 

 son with those which are linked together by 

 the author to make a consistent whole. This 

 would lead so far afield, however, that this re- 

 view will be held rather closely to a presenta- 

 tion of the vital points in each chapter, thus 

 placing stress on a summary of the arguments 

 of the ten chapters rather than on an analysis 

 of their bases upon which they rest. 



The first chapter is on the Gaseous Theory 

 of Earth-Genesis in the Light of the Kinetic 

 Theory of Gases. The spheroids of gravita- 

 tional control of the planets are considered, the 

 minimum radius of the earth's being about 

 1,000,000 kilometers. The relation of the mass 

 of the planet to its power to retain an atmos- 

 phere is the next thesis. Beyond that zone of 

 atmosphere which is dense enough to obey the 

 kinetic law of gases, but within the outer limits 

 of the spheroid of control, must lie an ultra- 

 atmosphere divided into two zones. The lower 

 of these is characterized by fairly free mole- 

 cules driven upward from below by the impacts 



