212 DISCUSSION OF THE DESICCATION QUESTION. 



It does not follow, however, that because the outlines of the existing con- 

 tinents were marked out at an early period in geological history, other con- 

 tinents might not also have been marked out, and then, after existing for a 

 time, have sunk beneath the ocean and disappeared forever. This indeed is 

 a very difficult and complicated question, which, even with all the assistance 

 rendered by recent investigations of the nature of the sea bottom, the depth 

 of the ocean, and the structure of the land masses, cannot as yet be positively 

 answered. There is no doubt, however, that the subject of the possible or 

 probable former changes in the ai^ea and distribution of the continents is one 

 of the greatest interest, not only from a purely geological point of view, but 

 from its connection with various inquiries with which physical geographers 

 are now occupying themselves. The study of the conditions and laws regu- 

 lating the development and distribution of plants and animals during the 

 successive geological ages is now one of the greatest possible interest, and 

 on tliis stad}^, as well as on the closely associated one of the phenomena of 

 climatic change during the successive epochs, the question of the stability 

 of the existing continents has a direct bearing. 



A careful consideration of the conclusions drawn by Dana, from flicts con- 

 nected with the geological structure of North America, with regard to the 

 stability of the ocean areas, seems to indicate that the land alone could not 

 furnish positive evidence on this point. What can be definitely made out 

 may be thus succinctly stated, as in part recapitulatory and in part explana- 

 tory of that which has been briefly set forth in the preceding pages. 



The framework of the existing continents was marked out at an early 

 period in the earth's history, and as it appears probable, by the development 

 in the earth's superficial layers, or crust, of certain areas having a tendency 

 to become more and more depressed below, and of others to become more 

 and more elevated above, a certain plane of reference, namely, the surface 

 of the oceanic waters, which connect with each other all around the globe, 

 and form what in point of fict is an arbitrary " bench mark," although none 

 the less a convenient and indeed indispensable one. Simplicity of theory, in 

 the absence of any positive information as to why these regions of depres- 

 sion and elevation have been thus selected and arranged with reference to 

 each other, leads us to the inference that what would appear to us an arbi- 

 trary interchange of position between these areas is not likely to have taken 

 place. This inference is still farther strengthened by the facts thoroughly 

 established in the course of the extensive series of deep-sea soundings made 



