ALTERATIONS OF LEVEL 67 



of the island or coast, particularly in connection with 

 its submarine slopes, can be understood. Certain 

 eftects recur on many lands, and the voyager soon 

 begins to recognize the nature of their rocks long before 

 he actually reaches their shores. Observations on 

 change of level are of importance, but difficult to make. 

 The earth has by no means a rigid or unbreakable 

 crust, and the present distribution of land and water 

 is generally considered to be due to the alterations 

 which have constantly been taking place in its surface 

 in past geological periods. The world must not be 

 considered as if these movements have now stopped. 

 Indeed, their cessation is directly negatived by the 

 existence of volcanic eruptions and earthquakes. 

 Seme movements, especially those associated with 

 volcanoes, are very local, but others affect wide areas, 

 such as the one which appears at the present time 

 to be slowly elevating the North of Scandinavia ; this 

 fir ds its balance in a general depression of the South of 

 England, there being thus a regular tilting over some 

 thousands of miles. This movement, fortunately, is 

 a slow one (as all large alterations of level probably 

 were), and has little effect on contemporary history ; 

 but many local upheavals and subsidences must have 

 taken place with great rapidity. There are many 

 lines and points of weakness in the earth's crust be- 

 neath the sea, and to the subterranean forces under 

 these we owe the existence of most of our oceanic 

 islands. The examination of the earth's crust beneath 

 the sea is impossible, so that special attention must 

 be paid to all signs of change of level round coasts on 

 octdanic islands. Elevation that is, the emergence of 

 formerly submerged land is shown by marine deposits 

 on the hill-slopes, which in the tropics are generally 

 largely calcareous in nature. Any appearances of 



