240 A. WINSLOW — GEOLOGY OF WESTERN ARKANSAS. 



dominant, and lateral erosion has advanced far. The valley in which Rus- 

 sellville is situated may also be included in this class. 



The Prairies of Western Arkansas. — Prairies are generally subordinate 

 valley features, and are properly mentioned here. They are not the broad 

 level tracts of country which are generally implied by the term prairie, and 

 of which the prairies of Kansas are familiar examples. They cover com- 

 paratively small areas. Their surfaces may be flat or undulating, or may 

 be diversified by knolls and small ridges. The absence or scarcity of trees 

 is the essential distinguishing characteristic. The prevalent underlying 

 rock of these prairies is always dark shale, which becomes exceedingly fissile 

 and soft on weathering. Thin strata of sandstone occur with this shale, and 

 these generally explain the existence and distribution of the various ele- 

 vations. The characteristically rounded contour of a soft shale country 

 rock is, however, dominant in the topography. A clayey soil exists fre- 

 quently to a depth of several feet, and this is apparently derived directly 

 from the shales by decomposition in situ. In places, however, the bare 

 shales are exposed immediately at the surface. The vegetation is chiefly a 

 coarse grass, which is high and luxuriant when not not cropped short and 

 trodden down by cattle. Clumps of small sassafras trees and haw bushes are 

 also often seen and, along the streams, willows, oaks, and other trees attain 

 full size. 



The maintenance of these prairies, and probably also their origin, may 

 be explained as due to a combination of causes ; namely, the alternation 

 from an extremely cold, wet soil during the rainy season, to a dry hard soil 

 in the dry season, and further the periodic recurrence of prairie fires which 

 shrivel such young tree growths as overcome the obstacles inherent in the 

 soils. In support of this hypothesis is the fact of the occurrence of small 

 bunches of trees over the small prairie knolls, which, being raised some- 

 what above the surface, have never so cold and water-soaked a soil as the 

 surrounding ground ; also the growth of trees along the drainage channels, 

 where there is more moisture in the dry season and where the conditions are 

 generally more uniform. Once started, however, a grow^th of trees continues 

 to flourish despite all the adverse conditions ; for Avith the commencement 

 of tree-growth the adverse conditions disappear. Hence it is that areas are 

 found which show evidence of having once been prairies but are now tim- 

 bered lands. Further, these prairies are now great open ranges for cattle, 

 the grass is kept short, and there are no longer such fierce periodic confla- 

 grations; consequently, in places, the growth of trees spreads rapidly over 

 the surface. 



A peculiarity of surface detail which often excites notice is the existence 

 of numerous small, low mounds which occur over these prairies, sometimes in 

 great profusion. In diameter they are generally under 50 feet, and in height 



