310 



EVOLUTIONAL GEOLOGY. 



building. It may be observed that at great depths readjustment will 

 be produced hy a slow flowing of .solid rock, and it is only compara- 

 tively near the surface, 5 or 10 miles at the most below, that failure of 

 support can lead to sudden fracture and collapse; hence the compara- 

 tively superficial origin of earthquakes. 



Given a sufliciently large coeflicient of expansion — and there is much 

 to suggest its existence (vide, p. 296) — and all the phenomena of moun- 

 tain ranges become explicable; thej'^ begin to present an appearance 

 that invites mathematical treatment; they inspire us with the hope that 

 from a knowledge of the height and dimensions of a continent and its 

 relations to the bordering ocean we may be able to predict when and 

 where a mountain chain should arise, and the theory which explains 

 them promises to guide us to an interpretation of those world- wide 

 unconformities which Suess can only account for by a transgression of 

 the sea. Finally it relieves us of the difliculty presented by mountain 

 formation in regard to the estimated duration of geological time. 



INFLUENCE OF VARIATIONS IN THE ECCENTRICITY OF THE EARTH's 



ORBIT. 



This may perhaps be the place to notice a highly interesting specu- 

 lation which we owe to Professor Bh'tt, who has attempted to estab- 

 lish a connection between periods of readjustment of the earth's crust 

 and variations in the eccentricity of the earth's orbit. ^Vithout enter- 

 ing into any discussion of Professor Blytt's methods, we may ofi'er a 

 comparison of his results with those that follow from our rough esti- 

 mate of 1 foot of sediment accumulated in a century. 



Table showing the time that has elapsed since the beginning of the systems in tliefi, st column, 

 as reckoned from thickness of sediment in the second column, and by Professor Blytl in 

 the third. 



It is now time to return to the task, too long postponed, of discuss- 

 ing the data from which we have been led to conclude that a probable 

 rate at which sediments have accumulated in places where they attain 

 their maximum thickness is 1 foot per century. 



RATE OF DEPOSITION OF SEDIMENT. 



We owe to Sir Archibald Geikie a most instructive method of esti- 

 mating the existing rate at which our continents and islands are being 



