EARTH-MOVEMENTS 229 



observations. We have yet to discover the different 

 rates of deposit, under the varying conditions in which 

 it is carried on in lakes, estuaries, and the sea. What, 

 for instance, would be a fair average for the rate at 

 which the lakes of each country of Europe are now 

 being silted up ? If this rate were ascertained, and if 

 the amount of material already deposited in these basins 

 were determined, we should be in possession of data 

 for estimating not only the probable time when the 

 lakes will disappear, but also the approximate date at 

 which they came into existence. 



But it is not merely in regard to epigene changes 

 that further more extended and concerted observation 

 is needed. Even among Subterranean movements 

 there are some which might be watched and recorded 

 with far more care and continuity than have ever 

 been attempted. The researches of Professor George 

 Darwin and others have shown how constant are the 

 tremors, minute but measureable, to which the crust 

 of the earth is subject. 1 Do any of these phenomena 

 indicate displacement of the crust, and, if so, what in 

 the lapse of a century is their cumulative effect on 

 the surface of the land? 



More momentous in their consequences are the dis- 

 turbances which traverse mountain-chains and find their 

 most violent expression in shocks of Earthquake. The 

 effects of such shocks have been studied and recorded 

 in many parts of the world, but their causes are only 

 partially understood. Are the disturbances due to a 

 continuation of the same operation which at first gave 

 birth to the mountains ? Should they be regarded as 



1 Report Brit. Assoc., 1882, p. 95. 



