20 CONCLUSIONS ON ANCIENT CLIMATES. 
been completely or even partially cleared of its forest growth, 
and brought under cultivation, the drying of the soil, under 
favorable circumstances, goes on for generations, perhaps for 
ages.* In other cases, from injudicious husbandry, or the diver- 
much of its coloring properties. —LAVERGNE, Economie Ruraie de la France, 
pp. 259-291. 
I believe there is no doubt that the cultivation of madder in the vicinity of 
Avignon is of recent introduction; but it is certain that it was grown by the 
ancient Romans, and throughout nearly all Europe in the middle ages. The 
madder brought from Persia to France, may belong to a diiferent 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 Petreea. 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 where 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 
ancient 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 ? 
For important observations on supposed changes of climate in our Western 
