208 DISCUSSION OF THE DESICCATION QUESTION. 



having been exerted synchronously, or nearly so, along a line perhaps thou- 

 sands of miles in length. These elevated portions of the land which have 

 furnished the detrital material have usually continued to do so during the 

 successive geological ages, as is shown by the fact that the grandest moun- 

 tain chains of the world have, spread out at their bases, or lying in crum- 

 pled folds on their flanks, the various members of the geological series, if not 

 in absolutely unbroken sequence, at least in something more or less nearly 

 approaching tluat condition. 



In view of these facts, it is evident that there must have been, on the 

 whole, a gain of the land on the earth's surface, unless it can be shown that, 

 in former ages, there have been one or more reversals of the existing con- 

 ditions of tendency to elevation and depression over different areas of the 

 earth's surface. In other words, the land must have been gaining in area 

 from the growth of the existing continents, unless other continents have 

 been disappearing at the same time to counterbalance the increase thus 

 produced. The superiority of the theory, from the point of view of sim- 

 plicity, that the oceanic basins, having once been begun by the earth's 

 shrinkage, would continue to remain areas of depression, v»ill not fail to be 

 recognized. Little as we really know of the precise mode of action of the 

 causes which have, during the successive geological agos, been at work to 

 raise certain portions of the earth's crust and to depress others, it certainly 

 throws additional difficulties in the way of our comprehending the process 

 of land-making and mountain-building, to have to admit that areas of eleva- 

 tion and depression have from time to time been arbitravil}' interchanged 

 as to their position. On the contrary, it appears decidedly more philo- 

 sophical to assume that areas of depression have always continued to be 

 such, and rice versa. What originally determined the relative position of 

 these areas, we know not ; but, having been marked out, their persistence 

 seems, if not a necessary result, at least more reasonable than the contrary 

 condition of things. 



The probability of the view of the persistence of the various regions of 

 elevation and depression has been greatly strengthened, since recent investi- 

 gations have proved so clearly how great the mean depth of the ocean is, 

 as compared with the mean elevation of the land. Recalling, in the first 

 place, the already mentioned fact of the great pieponderance of the surface 

 of the ocean over that of the land (the two being to each other nearly as 

 eleven to four), and considering that the mean depth of the ocean is now 



