216 DISCUSSION OF THE DESICCATION QUESTION. 



draw the inference that the body of land on the earth's surface has, on the 

 whole, been increasing during the geological ages. The main support of 

 this view is, however, to be found in the fact, demonstrated by geological 

 investigation, that the structure of the great mountain chains shows that 

 their upheaval has been the result of a long-continued process, during which 

 a tendency to rise has been counterbalancing the influence of tlie erosive 

 agencies always at work carrying down material from the rising areas to 

 form the stratified deposits on their flanks. Hence the highest chains are 

 usually those which contain the fullest series of the geological formations, 

 including the most recent as well as the oldest. It would appear that 

 greater and greater efforts had to be made, as time passed on, to rupture the 

 earth's crust, or at least to raise portions of it above the adjacent regions, 

 and that these efforts were, in the main, most successful either at the very 

 place where effects of the same kind had been previously produced, or else 

 in the immediate vicinity of it. If this had not been the case, new lines of 

 fracture having been started, the old areas of elevation becoming the station- 

 ary ones, these latter might have been entirelj^ swept away by erosion. 

 The effect of this would have been, in the long run, to bring the surface of 

 the earth, whether above or beneath the water, to a more nearly uniform 

 level, and at the same time to diminish, rather than increase, the entire area 

 of land surface on the globe. It is true that the ocean would be shallower 

 over certain regions adjacent to the land masses than it now is ; but this 

 diminished depth would not have a sensible effect on precipitation, since 

 evaporation goes on from the surface only. 



It may now be asked whether, having shown it to be highly probable, if 

 not certain, that there has been during the geological ages an increase of 

 land surface on the globe, we have entirely solved the problem before us. It 

 is admitted that such an increase would necessarily be followed by a diminu- 

 tion of the precipitation, unless counterbalanced by the effect of some other 

 agency working in the opposite direction. Here, then, we have a reasonable 

 cause for the desiccation which has been shown to be taking place over the 

 earth. The continents have become dry, because they have grown large. 

 The total amount of water evaporated has become smaller, because the sur- 

 face from which evaporation could take place has diminished. The interiors 

 of the land masses are insufficiently supplied with moisture, when not situ- 

 ated in the tropical belt, because their borders intercept the rain-bearing cur- 

 rents coming from over the warm and moist ocean surface and compel them 



