256 PROGRESS OF SCIENCE IN THE CENTURY. 



there is evidence of slow secular movements like 

 elevation. These latter movements are -usually 

 well marked in volcanic countries, and it is not un- 

 likely that the majority of earthquakes, even in vol- 

 canic countries, are the result of the sudden yielding 

 of rocky masses which have been bent till they have 

 reached a limit of elasticity. The after-shocks are 

 suggestive of the settling of disjointed strata." * 



It is probable, then, that while some earth- 

 quakes are due to subterranean explosions of steam 

 or other volcanic disturbances, the majority are due 

 to slips or fractures of the earth's crust in areas of 

 great strain. 



The improvement in the delicacy of earthquake- 

 measuring instruments (seismometers) has led to 

 a great extension of our knowledge in regard to the 

 diffusion of the undulations, and to a recognition of 

 the frequent minor tremors which would otherwise 

 have remained undetected. 



Crust-Movements. — It was in Scandinavia that 

 careful attention was first paid to those secular 

 changes of upheaval and depression, which, notwith- 

 standing their slowness, are more important geolog- 

 ically than either earthquakes or volcanoes. The 

 facts are particularly clear along the Scandinavian 

 coast, and even the fisher folk could not but be im- 

 pressed when they saw that the lines once cut to 

 mark sea-level became gradually more and more 

 inaccurate. Indeed the rise of land in Northern 

 Sweden has been estimated at as much as 2J feet in 

 a century. 



From Scandinavia the study of raised beaches 

 and unlifted shell-beds spread to Britain, and all 

 over the world. Evidences of depression were also 

 * Rep. Brit. Ass. for 1892, p. 128. 



