58 THE FIGURE OP THE EARTH 



marine sediments appear to be in almost all cases those 

 characteristic of shallow water. A large part of the 

 existing continental surface is not so far above the sea 

 but that a slight rise of the ocean-level would suffice to 

 submerge it. At the same time, even the shallow water 

 accumulations, out of which our mountain chains are to 

 a large extent built up, were deposited in sinking areas or 

 synclinals, which deepened pari passu with the progress 

 of deposition, till they attained a depth of 30,000 or 

 40,000 feet, or, singularly enough, that of the deepest 

 abysses of the ocean. These sinking areas, however, 

 were always situated near the margin of the sea, 

 and their distribution does not conflict with the course of 

 deformation as we have imagined it. Africa is distin- 

 guished from the other continents by a comparative 

 immunity from subsidence. The great mass of sediments 

 which form the larger part of the interior appear to be 

 continental deposits, i.e., laid down by fresh water over 

 an area of dry land. They contain no marine fossils, 

 and, so far, such fossils as they have afforded are remains 

 of reptiles and land plants. 



In providing an explanation of the origin of ocean 

 basins Mr. Jeans' hypothesis affords a welcome support 

 to the theory of mountain formation, as a consequence 

 of the contraction of a cooling globe ; for the opponents 

 of this theory have often sought to burden it with a task 

 probably beyond its powers, that is, of accounting for all 

 the inequalities of terrestrial relief, including the oceanic 

 depressions. These, however, as we see, are, in the 

 main, the result of the passage of our planet from one 

 form of equilibrium to another ; so that the effects of 

 cooling and contraction are only to be looked for in 

 comparatively subordinate features, such as compara- 

 tively late subsidences and mountain chains. The great 

 difficulty which in recent years has been urged against 



