HAS THE EXTENT OF LAND SURFACE BEEN INCEEASING? 217 



to discharge their contents before they have been carried far inland. This 

 effect would be produced in the case of very extensive areas of land, even 

 where portions adjacent to the sea were not considerably elevated above its 

 level ; but where, as is often the case, the borders of the continents rise in 

 lofty mountain chains, there, as has already been shown, the precipitation is 

 likely to be almost entirely cut off from the whole interior region. Again, 

 where the land is so situated that the winds are constantly blowing over it 

 in one direction and from a cooler towards a warmer region, these will be 

 desiccating winds, because their capacity for moisture is increasing as they 

 sweep over the country, and a lack of moisture is the inevitable result. 



With these general principles to guide us, we could, if we were able to 

 make out how the growth of any continental mass has taken place, form 

 some idea of the conditions prevailing over the region in question during 

 the successive epochs in respect to the distribution of precipitation. Thus, 

 taking into consideration the known effect of the Gulf of Mexico in miti- 

 gating what would otherwise, in all probability, be a sei-ious dryness in the 

 Mississippi Valley, we may infer, not without good reason, that when the 

 water covered an extensive portion of that valley, reaching, as it did in later 

 Tertiary times, up to the mouth of the Ohio and beyond, there must have 

 been a larger precipitation than now takes place over the adjacent region. 

 So, too, when the vast area east of the Ural Mountains was covered by the sea, 

 the ranges lying still farther eastward, in the line of the jDrevailing wdnds, 

 must have been favored with a moister climate than they now possess. 



Thus we might go on, and endeavor to ascertain what possible changes 

 have taken place in the forms of the land masses during the geological ages, 

 and especially the later ones, and in this manner be put upon the track of 

 the necessarily resulthig altei'ations in the character of the climates of the 

 regions thus subjected to changed conditions in the arrangement of land and 

 water in their vicinity. In doing this, however, we should have to assume 

 that there had been no agency of an opposite character at work, at the same 

 time, tending to bring about results of an opposite character from those indi- 

 cated as likely to be produced by the enlargement of the continents. This 

 might, indeed, have been the case ; but then, on the other hand, there may 

 have been other causes in action not antagonistic to the one dwelt upon 

 in the preceding pages, but in harmony with it, and thus lending their aid 

 to bring about a still more marked result than would have been produced by 

 orographic changes alone. 



