Auc.usT 8, 1907] 



A'.^ TURE 



359 



mcie hindrance than help 10 geologists ; its successors, 

 the nieteoritic hypothesis of Lockyer and the planctismal 

 theory of Chanibcrlin, are of far more practical use to 

 us, and they give a history of the world consistent with 

 the actual records of Geology. According to Sir Xorman 

 Lockvcr's nieteoritic hypothesis, nebula;, comets, and many 

 so-called stars consist of swarms of meteorites which, 

 though normally cold and dark, are heated by repeated 

 iclltsions, and so become luminous. They may even be 

 volatilised into glowing meteoric vapour ; but in time this 

 heat is dissipated, and the force of gravity condenses a 

 metcoritic swarm into a single globe. Some of the swarms 

 are, says Lockyer, " truly members of the solar system," 

 and some of them travel around the sun in nearly circular 

 orbits, like planets. They may be regarded as infini- 

 tesimal planets, and so Chamberlin calls them planetismals. 



The planetismal theorv is a development of the meteoritic 

 theory, and presents it in an especially attractive guise.' 

 It regards meteorites as very sparsely distributed through 

 space, and gravity as powerless to collect them into dense 

 groups. So it assigns the parentage of the solar system 

 to a spiral nebula composed of planetismals, and the 

 planets as formed from knots in the nebula, where many 

 planetismals had been concentrated near the intersections 

 of their orbits. These groups of meteorites, already as 

 solid as a swarm of bees, were then packed closer bv the 

 influence of gravity, and the contracting mass was heated 

 by the pressure, even above the normal melting-point of 

 the material, which w-as kept rigid by the weight of the 

 overlving layers. 



This theory has the recommendation of being consistent 

 with the history of the earth as interpreted by Geology. 

 For whereas the nebular hypothesis represents the earth 

 as having been originally intensely hot, and having 

 persistently cooled, yet geological records show that an 

 extensive low-level glaciation occurred in Cambrian times 

 in low latitudes in South Australia;' indeed, it seems 

 probable that, in spite of many great local variations, the 

 average climate of the whole world has remained fairly 

 constant throughout geological time. Whereas it has often 

 been represented, in accordance with the nebular theory, 

 that volcanic action has steadily waned, owing to the 

 lowering of the earth's internal fires and the constant 

 thickening of its crust, yet epochs of intense volcanic 

 action have recurred throughout the world's history, 

 separated by periods of comparative quiescence. Whereas 

 it has been assumed, as a corollary to the nebular theory, 

 that the force which uplifted mountain chains was the 

 crumpling of the crust owing to the contraction of the 

 internal inass, yet observation reveals that the crust has 

 been corrugated, and fold mountains formed by contraction 

 to an extent far greater than secular cooling can explain. 



{2^ The Matenah of the Inner Earth.— I'hts planetismal 

 hvpothesis is not only consistent with geological records, 

 but also with the known facts as to the internal compo- 

 sition of the earth and the structure of extra-terrestrial 

 bodies as revealed by meteorites. Meteorites are of two 

 main kinds — the meteoric irons, which consist of nickel 

 iron, and stony meteorites, which are composed of basic 

 minerals. Some of the stony meteorites have been 

 shattered into fault breccias, showing that they are frag- 

 ments of larger bodies which were subject to internal 

 movements, like those that have formed crush con- 

 glomerates in the crust of the earth. Those stony 

 meteorites, therefore, both in composition and structure 

 resemble the rocks in the comparatively shallow fracture 

 zone of the earth's crust. The nickel-iron meteorites, on 

 the other hand, represent the barysphere beneath the 

 crust. 



The earth appears to consist of material similar to that 

 of the two types of meteorites ; but whether the propor- 

 tions of the two materials in the earth represent their 

 proportions in other bodies and in meteoric swarms is 

 problematical. There appear to be no satisfactory data for 

 an estimate of the relative abundance in space of the iron 

 and stony meteoric material. .Stony meteorites have been 

 seen to fall far more frequently than iron meteorites ; 

 but the largest known meteorites are of the nickel-iron 

 group, although this material, in moist climates, very 

 soon decavs. The most trustworthy indication as to the 



^ As shown by the 



NO. 1971, 



vmk of Prof. Howchii 



'OL. 76] 



, of .Adelaide, 



relative amounts of the stony an.d nickel-iron meteorites 

 is given bv a comparison of the weight of the two types- 

 of material in meteorites of which the fall was seen- 

 According to Mr. Fletcher's list of the meteorites in the 

 British Museum up to 1904, the collection included 310)" 

 specimens of which the fall is recorded : of them 305 

 specimens were stony meteorites of an average weight of 

 2-63 lb., q were iron meteorites of an average weight of 

 2.3'i lb., and 5 were siderolites (or meteorites containing 

 a large proportion of both silicates and nickel iron) of an 

 average weight of 54 lb.' Therefore, according to this 

 test, the stony materials would appear to be the more 

 abundant. But if all known meteorites are considered, 

 the iron group far outweighs the other; for the iron 

 meteorites in the British .Museum collection weighed 

 11,873 lb., as against a total weight of only 865 lb. of 

 stony meteorites. The available evidence suggests that 

 the stony meteorites fall the more frequently on the earth, 

 but the meteoric irons come in such large masses that 

 they outbalance the showers of the smaller stones. 



We might have expected help from another source in 

 examining what lies below the .^rcha^an rocks. Cannot 

 the relative proportions of the stony and metallic con- 

 stituents in the earth help us? Unfortunately, this pro- 

 portion is as uncertain as that of stony and iron meteoritic 

 material. The best-established fact about the interior of 

 the earth is that its materials are much heavier than- 

 those of its crust. The specific gravity of the earth as 

 a whole is about 567 ; the specific gravity of the materials 

 of the crust may be taken as about 2-5, while that of 

 the heavier basic rocks is only about 30. Hence the 

 earth as a whole weighs about twice as much as it would 

 do, if it were built of materials having the same density 

 as those which form the crust. 



Two explanations of the greater internal weight of the 

 earth have been given. .According to one, the eai^h is 

 composed throughout of the same material, and the 

 internal mass is only heavier because it is compressed by 

 the weight of the overlying crust. Laplace estimated that 

 the material would gradually increase in density from the 

 surface to the centre, where its specific gravity would be 

 IO-74, and the calculations of Schlichter show that con- 

 densatioii due to compression may be adequate to account 

 for the greater internal weight. 



.According to the alternative or segregation theory, the 

 difference in density is explained as due to a difference 

 in composition ; the interior of the earth is thought to 

 he heavier owing to the concentration of metals within it. 

 The probability of this metallic interior has been advanced 

 from several lines of evidence ; and the assumed metallic 

 mass has received from Posepny the name of the" bary- 

 sphere," or heavy sphere. .According to this view the 

 earth is essentially a huge ball of iron, which, like modern- 

 projectiles, is hardened with nickel ; and it is covered 

 by a stonv crust, the materials of which were primarily- 

 separated 'from the metallic mass, like the slag formed 

 on a ball of solidifying iron in a puddling furnace. 



It has been objected that the weight of the earth is 

 not great enough for much of it to be composed of metallic 

 iron or of meteoritic material. The specific gravity of 

 iron under the pressure at the earth's surface is about 7-7, 

 and it would be even greater when compressed in the 

 interior. But the barysphere is doubtless impregnatecf 

 with much stony material that would lessen its weight. 

 An estimate by 'Farrington (1897') of the average specific 

 gravity of the meteorites of which the fall had been 

 recorded is only 3-69. .According to the Rev. E. Hilt 

 (1S85) the meari specific gravitv of all the meteorites in 

 the British Museum was 4-5 ; and, though Mr. Hill duly 

 considered the effect of compression, he concluded that 

 " the density of the earth Is perfectly consistent with its 

 being an aggregation of meteoric materials." Moreover, 

 within the metallic barysphere there may be a core of 

 lighter material ; for earthquake waves travel more slowly 

 iri the central core of the earth than in the intermediate 

 zone, or are even suppressed altogether there ; hence 

 the centre of the earth may be occupied by matter less 

 compact than that of the shell around it ; and, according 



t The weiehts nre pivn in pounds avoirdupois. ■ 'F'or the calculation T am 

 ind-hted to Mr. W. R. Wiseman, of the Geological Depattment of Glasgow 

 University. 



