250 Scientific Intelligence. 



6. The Evolution of Earth Structure ; by T. Mellakd Reade. 

 342 pp., 40 pis., 1903. (Longmans, Green & Co.) — In this vol- 

 ume Mr. Reade has brought together his views upon the causes 

 of crustal movements. The volume falls naturally into two 

 divisions, first the causes of broad vertical movements without 

 tangential thrust, commonly known as epeirogenic; and secondly, 

 the causes of tangential thrusts and their relations to mountain 

 building. 



However much geologists may differ from some of the con- 

 clusions of the work, the author's method must be regarded as 

 admirable, first presenting generally Conceded facts, secondly, 

 framing a hypothesis and, thirdly, showing its mechanical possi- 

 bility. Thus it may be said that Mr. Reade has developed true 

 causes, but it remains to be seen from further study and generali- 

 zations if they are quantitatively sufficient, or if there are other 

 more potent factors. Especially would possible changes of view 

 upon the nature of the earth's interior modify his conclusions. 



Taking up the first division of the volume, that concerning 

 vertical crustal movements, Reade cites familiar examples of 

 coastal oscillations and shows the incompetence of the principle 

 of isostacy to initiate such movements or to give them an oscil- 

 latory character. The author maintains that isostatic equilibrium 

 is true as a broad principle, as shown by the fact of the specific 

 gravity of the continental crust and subcrust being less than that 

 beneath the oceans, but relates this to diastrophic movements by 

 assuming that slight regional changes of volume but not of mass 

 occuf deep within the earth. 



Lateral shiftings within the earth would also result in such 

 movements, but no cause has been shown why within a solid earth 

 matter should so shift, especially as work would be done in lift- 

 ing the continental masses. In favor of the view that the move- 

 ments are due to slight volume changes without changes of mass 

 or lateral shifting, Reade discusses the irregular changes of den- 

 sity which take place during the cooling of a bar of iron and the 

 change in density due to magnetization. 



The author believes that most of the fluctuations of volume 

 take place within a depth of 500 miles from the surface in what 

 he terms the u sphere of igneous magma" and "the condition of 

 the matter may be normally solid, but potentially fluid, or actually 

 fluid when nearing the surface." Following this statement of 

 causes, the view is expressed from a consideration of the sea 

 coasts that the continents as a whole are at present in an era of 

 low level. An examination of the infrequency of deep sea 

 soundings and the discovery of occasional rapid variations in 

 depth leads to the further conclusion that there is merely nega- 

 tive evidence for the prevalent belief in the smoothness of the 

 ocean floor and the permanence of continents, but that in regions 

 removed from rapid sedimentation more careful soundings may 

 reveal bottoms which still show forms of subaerial erosion. A 

 considerable degree of impermanence of continents is further 

 shown by a consideration of basins of sedimentation, and as bearing 

 upon this problem two previous papers, "Denudation of the Two 



