MOVEMENT IN THE CRUST OF THE EARTH 3 



The islands of dry land have all been beneath the sea at 

 some time or other, and all show that they have been sub- 

 merged more than once, some more frequently than others. 

 During that portion of the history of the crust, which is the 

 theater of geological investigation, these periods of submarine 

 condition in one region always appear to be contemporaneous 

 with periods of subaereal conditions in some other region. Thus 

 there seem to have been regions of dry land and regions of 

 ocean bottom coexisting with a large predominance of oceanic 

 area. 



The aqueous envelope covers the rocky crust over about 

 three-fourths of its surface, and has an average depth of about 

 twelve thousand feet, though in extreme cases the bottom of the 

 sea is more than five miles below its surface, while in some few 

 cases mountains rise to more than five miles above the level of 

 the sea. It is certain that we are now able to study rocks 

 which were deposited at depths much greater than that of the 

 mean depth of the ocean, and there are many cases where 

 rocks found on the summits of high mountains are known to 

 have been deposited at great depths beneath the sea. Great 

 regions of country are at one time submarine, and at another 

 subaerial. These oscillations of upheaval and subsidence are 

 oft-repeated in geological history, and the swing of oscillation 

 seems to have been in some regions tens or scores of thou- 

 sands of feet where they reach the maximum, and to be only 

 tens or scores of feet at the minimum, so that the surface of 

 the earth, in so far as it has been studied geologically, is found 

 to give evidence of oscillations of level varying in these quan- 

 tities. 



These variations are geographically heterogeneous, one 

 region may have its oscillation on a small scale, another on a 

 large scale, the minor oscillations forming distinct geographical 

 series and the major oscillations forming distinct geographical 

 series ; that is, one region has been subject during geological 

 time only to minor oscillations, and another during the same 

 time to major oscillations. 



