558 



NA TURE 



[October 4, 1906 



set free by extrusive agency, and continues to be a 

 source of supply to tlie present time. The nature of 

 the primitive atmosphere may be conjectured from the 

 known occluded gases in crystalline rocks and 

 meteorites, having regard also to a certain selective 

 effect depending on molecular weights. Carbon 

 dioxide was probably abundant and nitrogen only a 

 minor constituent, the latter, in virtue of its chemical 

 inertness, having accumulated progressively through- 

 out subsequent time. It is supposed also that the 

 oxygen in the present atmosphere has mainly been 

 set free by the agency of vegetable life. The 

 initiation of vulcanism is next considered, involving a 

 discussion of the thermal conditions in the growing 

 globe. The heat produced by the infall of the planet- 

 esimals was probably important only in the earlier 

 stages of growth, and the chief source of the earth's 

 internal heat is ascribed to the progressive compression 

 of the central parts. It is estimated that this cause 

 alone would suffice to reach the melting temperature 

 of rock when the earth had acquired one-tenth of its 

 present mass. On account of the originally hetero- 

 geneous composition of the globe, local spots of fusion 

 ■would arise, the occluded gases presumably playing 

 a part in the process, and, aided by the varying differ- 

 ential attractions of the sun and moon, the molten 

 matter would gradually work its way outward. This 

 action is supposed to be facilitated by " selective 

 fusion," the more fusible materials encountered being 

 taken up and the more refractory of the old materials 

 deposited. In the general theory of igneous action 

 developed by the authors there is evidently much that 

 is debatable. In particular, the assumption that 

 minerals have their melting points raised without 

 limit by increased pressure, is one to which many 

 physicists will demur. The maximum melting point 

 found by Damien and others for various organic 

 bodies, and considered by Tamman to be a general 

 property, has led Arrhenius to very different con- 

 clusions concerning. the actual condition of the earth's 

 interior. 



Another part of our authors' system which fails 

 to carry complete conviction is the explanation offered 

 for the initiation of the ocean-basins. The cardinal 

 fact to be accounted for is the lower density of the 

 crust in the continental areas as compared with that 

 beneath the ocean floor. The difference is here 

 attributed to the weathering and leaching action on 

 the land, as contrasted with the relative protection of 

 the rocks under the sea. It is supposed that the 

 selective action of degradation and transportation sets 

 up in time an appreciable difference in composition 

 between the average material of the continental and 

 that of the suboceanic tracts, the former becoming 

 more acid and so lighter, and the latter more basic 

 and therefore denser. The effect would be cumulative, 

 and the difference of density established would be 

 permanent, not being obliterated by subsequent meta- 

 morphism. In this way there might be evolved, from 

 an originally fortuitous disposition of the growing 

 hydrosphere, a distribution of land and water having 

 a high degree of relative permanence. 



We have dwelt on that part of the work which 

 NO. 1927, VOL. 74J 



offers most of novelty, but the larger portion of the 

 two volumes deals, on a more familiar model, with 

 the several geological periods in order. The Archeean 

 era is regarded as representing the climax of igneous 

 action (or, as it is confusingly styled, volcanic action), 

 and as being concurrently a time of intense crustal de- 

 formation. The Huronian and other pre-Cambrian 

 formations which follow the Archsan are grouped as 

 Proterozoic — an unfortunate choice, since the name 

 has already been used by Lapworth for the Lower 

 Palasozoic. The Lake Superior region is taken as the 

 typical area, and three distinct systems are recognised 

 — Huronian, Animikean, and Keweenawan. The 

 great fossiliferous systems are then dealt with in turn, 

 the chief innovations as regards systematic arrange- 

 ment being the division of the Carboniferous into two, 

 Mississippian and Pennsylvanian, and the separation 

 of the Lower Cretaceous as a distinct system under 

 the name Comanchean. Under each head the develop- 

 ment of the stratified sequence in the North American 

 continent is described and its interpretation discussed, 

 the probable geographical conditions of the North 

 American area at different periods being illustrated by 

 maps. The corresponding strata of other parts of the 

 world are dismissed more summarily. This plan is 

 natural in a work designed primarily for American 

 students, and its inconvenience is felt only in certain 

 cases where the American record is incomplete or in- 

 adequate, especially in the Permian and Jurassic 

 periods. We have, however, as a digression, a good 

 account of the widespread glaciation in the southern 

 hemisphere in Permian times, with excellent figures 

 (after Schwarz) of glaciated rock-surfaces and boulder 

 deposits in South Africa. We think that the authors 

 have succeeded in giving a fairly complete and well- 

 proportioned sketch of the earth's history in its 

 successive chapters. The only serious defect which 

 we find is the slight notice accorded to igneous action, 

 and especially the failure (except in the earliest 

 chapters) to recognise this as an essential part of geo- 

 logical history, closely bound up with the tectonic 

 development of the globe. 



For reasons connected with the curriculum of 

 American universities, the history of life is treated in 

 great measure apart from the physical history of the 

 earth, a plan not without practical disadvantages. 

 No attempt is made to give a complete " roll-call " 

 of the flora and fauna of each period, but attention is 

 directed especially to the main lines of biological de- 

 velopment from the evolutionary standpoint. As re- 

 gards the evolution of life in general, it is supposed 

 that more than half of the complete history antedates 

 the first fair record, offered by the Cambrian strata, 

 in which we have abundant evidence of a development 

 already far advanced. For this reason the Cambrian 

 faunas are dealt with at some length. Similarly, in 

 the Carboniferous we have for the first time a large 

 mass of material bearing on the evolution of plant 

 life, and this receives due notice, with a digression 

 discussing the origin of coal and the climatic con- 

 ditions implied in the profusion of vegetable life at 

 that epoch. 



The arrangement of the book is in most respects 



