68 CHANGES OF LEVEL 



Hudson, whose ancient channel has been traced by means of 

 soundings for one hundred miles outside of Sandy Hook. 



A great thickness of shallow-water deposits is another obvious 

 proof of depression ; for if the sea- bottom did not sink, the water 

 would soon be filled up where it is shallow, and the coast-line 

 advanced. When we come to study the materials which are now 

 accumulating on the ocean floor, we shall learn how the depth of 

 water may be inferred from the character of the deposits. At 

 present it is only necessary to say that, from the Hudson River 

 southward, the Atlantic coastal plain is covered by a great thick- 

 ness of shallow-water beds, as revealed by the numerous artesian 

 wells which have been driven through it. A coast may also betray 

 recent subsidence by its form, but an account of this must be 

 deferred till we reach Part III. 



The Causes of Elevation and Depression. — Perplexing and 

 obscure as we have found the causes of the other subterranean 

 agencies to be, those which control the changes of level are far 

 more so, and no theory of their action yet propounded is at all 

 adequate or satisfactory. Perhaps the most generally accepted 

 view is that which brings them into relation with the secular con- 

 traction of the earth, due -to the slow shrinkage of the heated 

 interior, as it gradually cools, together with the flow of plastic 

 subcrustal material away from the lines of greatest pressure. This 

 theory is, however, very vague, and cannot be crucially tested. 



Another kind of movement of the crust of the earth is believed 

 to be that which follows upon changes of the load which any given 

 area has to carry. We shall see in later chapters that the rain, 

 wind, and rivers are continually cutting down the land surface and 

 carrying the materials thus obtained to the sea, where they accu- 

 mulate in great sheets. By some authorities it is believed that the 

 crust of the earth is in so exact a state of equilibrium, that any 

 area upon which a load of sediment is deposited will sink, while 

 one whence material is removed will rise because its load is light- 

 ened. This kind of adjustment is called isostasy, and its effect is 

 to preserve the inequalities of the earth's surface as they already 

 exist, while the general movements of elevation and depression 



