236 Part III. Chapter 3. 
coarse soil, for in some cases the deposits of coarse drift have escaped beit 
covered by the prairie soil, because a little more elevated in these spots 
the increased height has Keyahed the washing away of the finer particles b 
the rain. Then again where the continuity of the fine prairie soils is i 
rupted by a gulch or water course, by stream action, the closeness oft 
prairie sod is interrupted and trees appear. For this reason, if the heights 
the bluffs be considerable and the eroding power of the stream sufficiei 
cut the country up into a succession of ravines with but little level g 
between them then the whole region will be more or less covered with tin 
as is the case with northeastern Iowa, although the conditions with regard 
moisture are less favorable than in other parts of the state‘). 
Another interesting confirmation of this view is found on the E 
Plateau of Texas which is the rough deeply eroded southern margin 
Great Plains. If the plateau were an uneroded highland, climatic con 
remaining unchanged, the vegetal covering would be an open grass 
As a matter of fact it is being cut down to the coast level, as fast, as er 
can do it, and coincident with this, it is in process of transformation fr 
Similarly the invasion of trees in coast plain of Texas illustrates th 
pendence on edaphic conditions. The soils here consist of compact clays 
silts interspersed with areas having a larger proportion of sand, and, there 
more porous soils. These latter were more easily captured by forest 
compact soil more slowly. Thus one finds alternating areas of forest 
prairie °). 
The grass formation or prairie formation is clearly older floristical 
king than the forest groves and forest areas along the streams. As 
shown subsequently, the grass formations are endemic, while t 
trees have been derived in the main from the great southeastern forest f 
ously: described, while part of them have entered the North American 
district from the west. The reason why the invading trees extend west 
and northwestward into the prairie country from the Mississippi valley pfO 
along which they have migrated, thus has a reasonable physiographi 
climatic explanation. 
The distribution of the timbered and prairie tracts in Wisconsin 
the dependence of the forest growth on the geologic and historic ( 
rather than those having to do with climate. In the northern part of‘ 
is a region of dense forest, although as the statistics of rainfall show | 
not a region of large precipitation. It is, however, covered with coars 
trital matter, plentifully distributed from the headquarters of the drift on 
ı) WHITNEY, J. D.: Plain, Prairie, Forest, American Naturalist X: 577, 
656. 
2) BrAv, W. L.: Forest Resources of Texas. Bureau of Forestry U. S. Pape be 
eulture Bulletin No. 47.: 1904: 8 and 28.. 
