230 



The Realm of Nature 



CHAP. 



the same fate and passed on the process. Some geologists 

 believe that as denudation lightens the ridges and loads the 

 hollows, the Earth's crust is strained by the redistribution 

 of the pressure on it ; that consequently the strata snap with 

 a succession of earthquake shocks, and the parts loaded 

 with deposits sink, while those lightened by the effect of 

 erosion are upraised. Other geologists take an opposite 

 view of the result of sedimentation ( 304). The typical 

 form of an Earth wrinkle is a gentle ridge, A, accompanied 

 by a gentle hollow, S (Fig. 45). The curved strata of the 



FIG. 45. Strata bent into anticline A and syncline S. 



ridge are said to form an anticline, because at the summit A 

 the strata, as shown by the arrows, dip or incline away from 

 each other. The curved strata of the trough are similarly 

 said to form a syncline, as at S the strata dip together 

 or toward each other. Even although the wrinkled crust 

 should be worn smooth by erosion to form the surface ss', 

 it is still easy to tell by observing the dip of the strata 

 where the ridge and the hollow were situated. Thus rock 

 structure is not concealed by surface change. Synclines 

 and anticlines are ridged up in consequence of the lateral 

 pressure or tangential thrust produced by the downsink- 

 ing of part of the crust. The tremendous lateral pressure 

 effected by a great subsidence throws the strata on both 

 sides into sharp anticlines and synclrnes, while at a greater 

 distance from the origin the wrinkles are low and uniform. 

 The Geological Survey of Scotland has brought to light 

 many remarkable proofs of the intensity of the thrust 

 which ridged up the western margin of Europe in ancient 

 times. Sometimes the compressing force was so violent 

 that the strata, instead of puckering up into anticlines and 



