3 60 THE PLAN OF THE EARTH AND ITS CAUSES. 



map. The antipodes of a point in the center of the continent of North 

 America occurs in the Indian Ocean; and if we mark on a map the 

 antipodes of all the points in North America, we should find that the 

 whole of that contiuent is exactly antipodal to the Indian Ocean. 

 Similarly, the elliptical mass of Europe and Africa is antipodal to the 

 central area of the Pacific Ocean; the comparatively small continent 

 Australia is antipodal to the comparatively small basin of the North 

 Atlantic; the South Atlantic corresponds — though less exactly — to the 

 eastern half of Asia; and the Arctic Ocean is precisely antipodal to 

 the antarctic land. 



These, then, are the three fundamental facts in the existing plan of 

 the globe. Our main problem is, Why are the geographical elements 

 thus shaped and thus distributed? 



THE EARTH'S CONCENTRIC SHELLS. 



It simplifies the statement of the problem to remember that the 

 earth consists of three parts: There is the vast unknown interior, or 

 " centrosphere," concerning which physicists have not come to any 

 unanimous decision, some saying that it is throughout solid and 

 rigid, others that it is partly fluid, and others again that it is partly 

 gaseous. This interior mass is inclosed by a shell formed of two layers, 

 the solid crust, or "lithosphere," and the oceanic layer, or "hydro- 

 sphere." It is possible that at first the two layers of the shell were 

 regular and uniform, in which case the whole world was covered by a 

 universal ocean ; but before the dawn of geological history this arange- 

 ment had been disturbed by the formation of irregularities in the 

 surface of the lithosphere. Dry land appeared at the areas of eleva- 

 tion, and the waters gathered together into the intervening depressions. 



The problem, then, of the distribution of land and water on the 

 globe is the problem of the distribution of irregularities in the surface 

 of the lithosphere. We are accordingly at once brought face to face 

 with the question. When were the existing irregularities made? If, as 

 many authorities say, these depressions date from the earliest days of 

 the earth's history, and have lasted unchanged in position throughout 

 geological time, then we are thrown back upon some cause which 

 acted when the earth was in its infancy. In that case the question is 

 astronomical and physical, instead of geological and geographical. 



PRE-GEOLOGTCAL GEOGRAPHY. 



There have been several attempts to solve the question astronom- 

 ically, of which the most important is that of Prof. G-. H. Darwin. 

 According to his luminous theory the tidal action of the sun on the 

 viscous earth formed two protuberances at opposite points of the 

 equator; one of the protuberances broke away and solidified as the 

 moon, which revolved round the earth much nearer than at present. 

 Asa new equatorial protuberance formed the moon pulled it backward, 

 thus causing a series of wrinkles in the earth's crust, which persist as 



