240 PROGRESS OF SCIENCE IN THE CENTURY. 



that the amount of contraction was altogether inade- 

 quate to explain the wrinkling, but Prof. Sollas finds 

 sufficient flaws in the data to warrant him in still 

 maintaining the theory of contraction. " The con- 

 traction of the interior of the earth, consequent on 

 its loss of heat, causes the crust to fall upon it in 

 folds, which rise over the continents and sink under 

 the oceans, and the flexure of the area of sedimenta- 

 tion is partly a consequence of this folding, partly 

 of overloading." * 



Another factor may be chiefly alluded to. Since 

 the floor of the ocean has a temperature about zero, 

 and is some three miles below the continental level, 

 surfaces of equal internal temperature will not be 

 spherical, but will rise beneath the continents and 

 sink beneath the ocean, and the effect will be to ren- 

 der the continents mobile as regards the ocean floor ; 

 or vice versd (Sollas). 



We have cited enough to illustrate a kind of in- 

 quiry eminently characteristic of the end of the 

 nineteenth century which the new century is certain 

 to develop to more stable and precise results. 



The general result may be summed up in a sen- 

 tence; the contraction of the interior probably ac- 

 counts for much of the folding and crumpling of the 

 exterior; other physical factors are and have been 

 at work; and the transforming influences of water, 

 of the atmosphere, and of life have been continuous 

 and momentous since they first began to act. 



It must not be supposed that the evolution-idea in 

 Geology has been restricted in application to the 

 recondite problem of the Earth's early phase* ; the 

 idea has influenced the whole science and is illus- 

 trated in the modern treatment of river-development, 

 or of coral reefs, or of details of scenery, and so on, 

 Solla*. toe. cit. 



