324 BOTANY AND PALEONTOLOGY 



alluvium, as in some marshes underlaid by clay or shales, or along the 

 banks of some rivers. Land continually covered with stagnant water 

 cannot produce any trees, because the trees require for their growth, like 

 most of the terrestrial plants, the introduction of atmospheric air to their 

 roots. Neither do trees germinate and grow on a ground alternately 

 covered with stagnant water and exposed to dryness for some months of 

 the year. From these considerations, the law of the general formation 

 of the prairies can be deduced : While a land or a part of a country is 

 slowly passing from the state of swamp or marsh to the state of dry land 

 the annual alternative of stagnant water and dryness causes the vegetation 

 of peculiar plants, which, by their decomposition, form a peculiar soil un- 

 favorable to the growth of the trees. From this general rule of formation, 

 which regards only the prairies of the Mississippi valley,* all the different 

 phenomena or peculiar appearances of the prairies can be easily explained. 



The prairies of Arkansas, following their vegetation and their geological 

 connection, may be separated into three classes : 



1st. The prairies of the North, mostly underlaid by cherty limestone. 



2d. The prairies of the "West, on carboniferous shales and clay. 



3d. The prairies of the South and East, overlying tertiary and alluvial 

 formations. 



1st. The limestone prairies of North Arkansas mostly belong to the 

 counties which are examined in the next division. They are singular in 

 this fact, that their surface is not always flat, and that they are mostly 

 placed on soft declivities or coves along or between the ridges. They are 

 mostly of small extent and surrounded by thickets of low trees. The 

 compact or somewhat porous Subcarboniferous Limestone which they cover 

 does not absorb water with rapidity. Hence, in the spring, water perco- 

 lates slowly along the slopes, taking with it the detritus of the stone, and 

 depositing it where its course is either stopped or slackened. A scant 

 swamp vegetation springs up there, its decomposed remains are mixed 

 with the original deposit, which, by and by, augments in thickness under 

 the action of water and of vegetation. This soil is naturally spongy, pre- 

 serves water for a part of the year, like the peat, which it resembles, and 

 thus cannot sustain trees. They establish themselves on a firmer ground 

 all around. "When by successive contribution of limestone deposited by 

 water and of particles of humus received from the plants this soil has 

 become thick enough, it is, when drained by a few ditches (serving as 

 channels for the water of the rainy season), a fertile and easily cultivated 

 ground. The channels of drainage are generally formed by a natural 

 depression, the depth of which varies with the thickness of the soil of each 



* The prairies of the far West, along the eastern base of the Rocky Mountains, are true sandy 

 deserts, caused by the dryness of the atmosphere. 



