4 INTRODUCTION. 



Although here and there over hmited areas we find evidences 

 of great and apparently abrupt physical changes, we can 

 detect no signs of any universal catastrophe : on the contrary, 

 through every epoch we discern a Uniformity in cause not 

 only in the inorganic world, but also in the organic world, 

 accompanied though it be (on the one hand) by evidences of 

 evolution, or the gradual development, in the course of time, 

 of higher and higher forms of life ; and (on the other hand) by 

 the local catastrophic action of earthquakes and volcanoes. 

 In these questions we cannot of course limit our observations 

 to the British Islands ; nor must we, in concluding that the 

 ph)'sical forces have been the same throughout geological 

 time, -suppose that their action has been always of similar 

 intensity to that of which we have definite proof in the 

 present. Every volcanic outburst, indeed, is a loss of energy. 



Thus while the greater portion, if not the whole, of the 

 present land has been at times submerged beneath the ocean, 

 we cannot prove that all oceanic areas have at times been 

 dry land. This may have been the case, notwithstanding the 

 conclusions of recent investigators, who favour the view that 

 the ab\-smal regions of the ocean have been the more per- 

 manent areas of the earth's surface.^ 



The causes of great physical changes are intimately con- 

 nected with the internal structure of the earth. The Rev. O. 

 Fisher is of opinion that it consists of a rigid nucleus, nearly 

 approaching the size of the whole globe, covered by a fluid 

 (molten) substratum of no great thickness compared to the 

 radius, upon which a crust of lesser density floats in a state of 

 equilibrium. This crust he concludes to be about twenty-five 

 miles in thickness ; and being in this unstable condition, any 

 considerable load added to it will cause a region to sink, or 

 any considerable amount of material removed will cause an 

 area to rise.^ This hypothesis helps to explain the changes 

 of level revealed in geological history, for it indicates that the 

 great accumulations of sediment may be one cause of the 

 subsidence of an area, and the gradual waste of land and the 

 transfer of the material to other tracts, one cause of the 

 upheaval of land. At the same time we must not neglect 

 the aid of subterranean changes, whether in active volcanic 

 action or in the rupture of the earth's crust on shrinkage and 

 contraction ; to which causes the more important instances 

 of elevation and depression were probably due. 



1 J. Murray (Brit. Assoc. 18S5), Nature, Oct. 22, 18S5, p. 613. 

 - Physics of the Eartli's Crust, pp. 18, 269, 275, 286 ; see also Dr. C. Ricketts 

 (who first drew prominent attention to this subject), G. Mag. 18S3, pp. 302, 34S. 



