NATURE 



557 



THURSDAY, OCTOBER 4, 1906. 



THE EVOLUTIOX OF THE GLOBE. 

 Geology: Earth History. By Thomas C. Chamberlin 

 and RoIIin D. Salisbury. Vol. il., Genesis, Paleo- 

 zoic, pp. .\xvi + 6y2. \o]. iii., Mesozoic, Cenozoic, 

 pp. xi + 624. (London: John Murray, 1906.) Price 

 J I.?, net each. 



THK first volume of this imporl;int work, noticed 

 in Naitrf. of Jrmuary 19, 1905, and already in 

 its second edition, dealt with geological processes and 

 their results. In the two volumes now before us, 

 which complete the work, geology is treated from 

 the historical side, and we have a comprehensive re- 

 \iew of the history of the earth on systematic lines. 

 The treatment of these two formally separable 

 branches of the science is, however, such as to 

 emphasise the essential unity of the whole. As geo- 

 logical processes were discussed with continual refer- 

 I'Tice to the historical application of the principles laid 

 down; so the evolution of the globe, which is the 

 story of these latter volumes, is regarded consistently 

 from the causal point of view. Indeed, some subjects 

 already considered under the head of geological pro- 

 cesses, such as the dynamics of deformation, the 

 causes of glaciation, &-c. , are now more fully discussed 

 in connection with the particular geological periods 

 which most clearly exemplify the phenomena. 



The part of the work which will be read with 

 greatest interest is that which falls under the subtitle 

 "Genesis." Considering geology as "the domestic 

 chapter of astronomy," the authors devote much more 

 space than is customary in geological treatises to the 

 problem of the origin and primitive condition of the 

 globe. This is, we think, amply justified by the 

 fundamental place which cosmogony necessarily 

 occupies in the construction of the science. It is 

 evident that opinion concerning such questions as the 

 causes of crust-movements, the essential mechanism 

 of igneous action, the origin of the atmosphere and 

 hydrosphere, the beginning of life, must be controlled 

 by the view adopted, formally or tacitly, of the mode 

 of origin of the earth as a planet. Less obviously, 

 but not less surely, some theory of the earth's initial 

 state is involved in numerous geological doctrines, the 

 dependence of which on such considerations is liable 

 to be overlooked ; and the authors do good service in 

 recalling this fact repeatedly in the historical record 

 which follows. The clear recognition of cosmogony 

 as the foundation of geology, by revealing an un- 

 suspected element of hypothesis at various places in 

 the superstructure, offers a warning which is perhaps 

 in some quarters not wholly unnecessary. 



The special interest of this part of the book, how- 

 ever, lies in the first complete exposition of the 

 " planetesimal " theory, which the senior author has 

 already propounded elsewhere. That our solar system 

 has in some manner been evolved from a nebula of 

 some kind is an assumption to which few will demur ; 

 but the particular theory associated with the name of 

 Laplace, and generally known as the nebular hypo- 

 thesis, starting from a gaseous nebula of extreme 

 NO. 1927, VOL. 74] 



tenuity, has for some time been felt to involve difficul- 

 ties, which become more serious upon a closer examin- 

 ation. These difficulties are cogently stated by the 

 authors, especial stress being laid on the great dis- 

 crepancy which Moulton has pointed out from a 

 consideration of the actual distribution of moment of 

 momentum in the solar system. The metcoritic 

 hypothesis, whether in Lockyer's or in Darwin's form, 

 is held by the authors to be open to the same objec- 

 tions as the theory of a gaseous nebula, with which, 

 indeed, it is practically identical as regards its more 

 important consequences. According to the planet- 

 esimal hypothesis, the constituents of the system 

 might be molecules or small masses of any kind 

 moving in orbits about a common centre, the essential 

 point being that their behaviour depended, not on 

 mutual collisions (as on the meteoritic hypothesis), 

 but on revolution in independent orbits. On this 

 supposition there was, after the initial nebula was 

 once formed, no fundamental change in the dynamics 

 of the system, but only a progressive aggregation of 

 the infinitesimal planetoids (" planetesimals ") to 

 form the planets and their satellites as they now 

 exist. 



The original nebula postulated was not a gaseous 

 one, but belonged to the type giving a continuous 

 spectrum, and had, like most of these, the spiral 

 form. There were also, as in such nebulae in general, 

 knots of denser aggregation which became the nuclei 

 of the several planets, though the greater part of the 

 material outside the central heboid was still widely 

 scattered. The manner in which such a system may 

 have been developed from an ancestral helioid by the 

 near approach of another star is tentatively pictured ; 

 but this is no essential part of the hypothesis, which 

 is concerned, not with the whole evolution of the solar 

 system, but with the birth and subsequent history of 

 the planets. Starting with the conception of an in- 

 finitude of small masses revolving in different elliptic 

 orbits of considerable eccentricity, with a certain 

 degree of clustering already pronounced, the authors 

 discuss the manner in which these planetesimals be- 

 came aggregated into planets, moving in orbits of 

 only small eccentricity, and with rotation in the same 

 direction as the orbital revolution. 



It is for the mathematician rather than the 

 geologist to pass judgment upon this new treatment 

 of the dynamical problems involved, but the geologist 

 must be vitally interested in the verdict. The earth 

 as built up on the planetesimal hypothesis will be a 

 very different body from the earth as condensed from 

 a gaseous spheroid, and must have passed through 

 very ditTerent stages of evolution since it acquired 

 individuality. The first-formed solid nucleus was 

 probably devoid of any atmospheric envelope, its 

 attraction being insufficient to control the rapidly 

 moving molecules of gases. An appreciable atmo- 

 sphere had probably been gathered when the growing 

 globe had attained one-tenth of its present mass (being 

 then comparable with Mars). The atmosphere would 

 at first be collected from outside, but there was already 

 a large quantity of occluded gases in the material 

 built into the solid globe,^ which might eventually be 



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