CONCLTJSIONS OIT ANCIENT CLIMATES. 19^ 



ally contains. After a district of country has been completely or 

 even partially cleared of its forest growth and brought under cul- 

 tivation, the drying of the soil, under favorable circumstances, 

 goes on for generations, perhaps for ages.* In other cases, from 



Avignon is of recent introduction ; but it is certain that it was grown by tho 

 ancient Romans, and throughout nearly all Europe in the middle ages. The 

 madder brought from Persia to France, may belong to a different species, or 

 at least variety. 



* In many parts of New England there are tracts, many square miles in ex- 

 tent and presenting all varieties of surface and exposure, which were partially 

 cleared sixty or seventy years ago, and where little or no change in the pro- 

 portion of cultivated ground, pasturage and woodland has taken place since. 

 In some cases, these tracts compose basins apparently scarcely at all exposed 

 to any local influence in the way of percolation or infiltration of water towards 

 or from neighboring valleys. But in such situations, apart from accidental 

 disturbances, the ground is growing drier and drier from year to year, springs 

 are still disappearing, and rivulets still diminishing in their summer supply of 

 water. A probable explanation of this is to be found in the rapid drainage 

 of the surface of cleared ground, which prevents the subterranean natural 

 reservoirs, whether cavities or merely strata of bibulous earth, from filling up. 

 How long this process is to last before an equilibrium is reached, none can 

 say. It may be, for years ; it may be, for centuries. 



Livingstone states facts which strongly favor the supposition that a secular 

 desiccation is still going on in Central Africa, and there is reason to suspect 

 that a like change is taking place in California. When the regions where the 

 earth is growing drier were cleared of wood, or, indeed, whether forests ever 

 grew there, we are unable to say, but the change appears to have been long in 

 progress. A similar revolution appears to have occurred in Arabia Petrsea. 

 In many of the wadis, and particularly in the gorges between Wadi Feiran 

 and Wadi Esh Sheikh, there are water-worn banks showing that, at no very 

 remote period, the winter floods must have risen fifty feet in channels whero 

 the growth of acacias and tamarisks and the testimony of the Arabs concur to 

 prove that they have not risen six feet within the memory or tradition of the 

 present inhabitants. Recent travellers have discovered traces of extensive an- 

 cient cultivation, and of the former existence of large towns in the Tih desert, 

 in localities where all agriculture is now impossible for want of water. Is 

 this drought due to the destruction of ancient forests or to some other cause ? 

 L'Annee Oeographique for 1873, pp. 72 and 176, has some very interesting ob- 

 servations on the secular desiccation of the Sahara and of Persia. 



For important remarks on supposed changes in our Western prairie region, 

 from cultivation of the soil and the introduction of domestic cattle, see Bry- 

 ant's valuable Forest Trees, 1871, chapter v., and Hayden, Preliminary Beport 

 on Survey of Wyoming, p. 455. 



Some physicists believe that the waters of our earth are, from chemical or 

 other less known causes, diminishing by entering into new inorganic combi- 

 nations, and that this element will finally disappear from the globe. 



