FORESTS AND TREES OF NORTH AMERICA. 277 



less rain than is necessary for most trees to thrive, and in some years 

 there is even a greater supply of rain in the Texan and Illinois regions 

 than eastward. But there are years and series of years of drought, 

 when in tlieir natural condition the forests take fire from the slightest 

 cause and burn over large tracts. This was made even more general 

 by the Indians, but since the white settlement has in great degree 

 ceased and forests have been re-established. In the Apalachian re- 

 gion droughts have never been sufficient to keep trees from extending 

 themselves as soon as a forest might be partially destroyed by fire, 

 and thus the formation of prairies has been prevented. A consideration 

 of the source of the rains will explain why the limit of prairies has its 

 present direction. Coming north from the Gulf they are continually 

 carried more and more eastward by the westerly winds; and as the 

 greater part of the moisture is precipitated before reaching the Ohio 

 river, the Illinois region is deprived /or 7/ian^ years of its due share of 

 rains. 



The Texan region lying quite west of the line of travel of those Gulf 

 streams has to depend on less abundant sources for its rains. Now, as 

 we go westward the supply rapidly diminishes until in the Camanche 

 and Dacotah regions it is entirely inadequate to the growth of trees as 

 well as of most cultivated products; and in some parts even grass and 

 other herbage entirely disappear over vast tracts. From the great 

 bend of tlie Missouri north, however, there seems to be an improve- 

 ment in the country. On the banks of that river, above Fort Union, 

 there is no long interval without trees as there is farther south on 

 nearly all the streams, and on the Saskatchewan there is even less. 



The very porous character of the soil and underlying rocks assists 

 much in this aridity of the country, and we therefore find that the 

 line marking the junction of the carboniferous rocks of the Illinois re- 

 gion with the cretaceous and tertiary is a distinct limitation of many 

 trees. 



When better known the geological character will help much in de- 

 fining the physical geography of the surface of this province. In 

 Texas the border of the Llauo Estacado coincides with that of the Ca- 

 manche region for a long distance. It is evidently more the reten- 

 tiveness of the soil than its mineral composition that affects the growth 

 of trees, for all soils contain more or less of their essential ingredients. 



Even the saline substances, which are supposed by some to make 

 deserts of portions of the Great Plains, are rather the secondary effects 

 of the climate; for if rains were abundant these salts would become 

 difi"used, and in their proper proportions enter into the structure of trees 

 and other plants. 



It is certain, however, that even if the fires cease very few trees 

 will ever be made to grow in these two arid regions. 



5. Coming now to what I have called the Rocky Mountain province, 

 we find that the relations of climate and forest characteristic both of 

 the Campestrian and Apalachian provinces are repeated, but com- 

 bined in an entirely new manner. 



The high mountain ranges resemble the latter in their regular sup- 

 ply of rain, while near their summits the vegetation of the Athabascan 

 region appears either in identical or allied forms, and still higher, 



