BOTANY AND PALAEONTOLOGY OF ARKANSAS. 321 



country than can chemical constituents of the rocks. Consequently, that 

 the direction of the ridges, the amount of light and atmospheric heat and 

 moisture, the thickness of the soil, its hardness and capacity for retaining 

 water, are the essential causes of the distribution of the plants. 



These considerations may be true, but they touch only one side of a 

 complex and difficult question which cannot be discussed now. If the 

 hardness, compactness of a soil, its capacity for retaining water and heat, 

 are essential causes affecting the distribution of the plants, it is evident 

 that this cause depends principally on the chemical nature of the geolo- 

 gical strata. On the other hand, if the dissemination of the geological 

 elements renders the task of ascertaining their influence difficult in some 

 places, it is not a reason to reject as useless or impossible any attempt to 

 compare the vegetation of a country with its geological formations. If 

 this comparison can be made anywhere with a chance of success, it is cer- 

 tainly in Arkansas, where the strata are nearly horizontal and extend over 

 vast areas. 



The exploration of the Botany of Arkansas began too late and was too 

 short to permit the fulfilling of a work which for its completion would 

 require some years of continual research. The following data collected 

 along our road of travel can thus be considered only as the first points of 

 delineating lines which may be continued and completed hereafter. 



MAMMOTH SPRING OF FULTON COUNTY. 



The Mammoth Spring of Fulton County has been already described on 

 page 60 of the first volume of this Eeport. Its water is almost entirely 

 filled with aquatic plants covering its bed even to a great depth or floating 

 on the surface. A phenomenon like this, in a spring of so wide an extent, 

 is remarkable enough to merit an examination. 



It is well known that plants absorb by the green surface of their leaves 

 a certain amount of carbonic acid which serves them as food, and which 

 they transform into carbon. Springs emerging from limestone rocks 

 generally contain carbonic acid in small quantity and thus may nourish 

 some plants in their water. 



The water of the Mammoth Spring, either by compression or from some 

 other peculiar cause, contains, apparently, in solution, such a great amount 

 of carbonic acid that its surface is in a continuous state of effervescence 

 or bubbling, resembling the effervescence of a fountain of soda water. 

 Perhaps the phenomenon is caused by atmospheric air taken into the water 

 by its running through beds of porous cherty limestone from which it 

 emerges. This would not alter the conclusion, because atmospheric air 



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