I04 
The American Geologist. 
February, 1901. 
easily be illustrated by a plain map. The antipodes of a point 
in the center of the continent of North America occurs in the In- 
dian 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 
continent 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 con- 
tinent Australia is antipodal to the comparatively small 
basin of the North Atlantic ; the South Atlantic corresponds — 
FIG. I. — MAP OF THE WORLD, SHOWING THE 
DISTRIBUTION OF ANTIPODAL AREAS. 
though less exactlv — 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 geo- 
graphical 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 in- 
terior, or "centrosphere," concerning which physicists have not 
come to any unanimous decision, some saying that it is through- 
out solid and rigid, others that it is partly fluid, and others again 
that it is partlv gaseous. This interior mass is enclosed by a 
shell fomied of two layers, the solid crust, or "lithosphere," and 
the oceanic layer, or "hydrosphere." It is possible that at 
first the two layers of the shell were regular and uniform, 
