242 ANNUAL OF SCIENTIFIC DISCOVERT. 



ORIGIN OF THE PRAIRIES. 



Col. J. W. Foster read a paper, at the 1868 meeting of the Amer- 

 ican Association, on this subject. He said that, wherever we 

 examine a continental mass, we ordinarily JBnd a wooded belt 

 along the shores, succeeded as we advance inland by grassy 

 plains, and graduating in the interior into inhospitable deserts. 

 Whenever we study the annual precipitation of moisture, in con- 

 nection with the laws of temperature, we find that, wherever the 

 moisture is equable and abundant, we have the forest ; wherever 

 it is unequally distributed, we have the grassy plains ; and, where- 

 everitis mostly withheld, we have the inhospitable desert. The 

 varying supply of moisture, then, is sufficient to account for the 

 diversity of vegetation modified to some extent by the physical 

 features of the country, altitude above the sea, and the extremes 

 of heat and cold. A distinguished botanist has undertaken to 

 trace analogies between the formation of the prairies and that of 

 the peat swamps, in the first volume of the Illinois geological re- 

 ports. It was a theory which presupposed a humid climate, a 

 level country, with imperfect drainage, and with a surface dotted 

 over with lakes and sheltered from the winds, where the peat-pro- 

 ducing plants could grow, — conditions none of which obtain 

 where the prairies assume their grandest proportions. It is evi- 

 dent, therefore, that we must resort to other and different causes 

 to explain the phenomena of these grassy plains. Other physi- 

 cists would attribute the formation of the prairies to the mechanical 

 or chemical composition of the soil, — a theory equally untenable, 

 when we reflect that the surface of these treeless plains may vary 

 in every degree between drifting sands and impervious clays, and 

 that the efflorescences of soda and gypsum which are the eviden- 

 ces of an arid climate at one extremity of the continent would 

 become fertilizing agents at the other. The theory, very much in 

 vogue before the laws of climatology were fully understood, which 

 attributes the formation of the prairies to the annual fires lit by 

 the Indians, is deserving only of passing notice. If these regions 

 were once wooded, we should expect to find the remains of an 

 arborescent vegetation entombed in the sloughs, where they would 

 be capable of indefinite preservation. If their treeless character 

 was due to the annual fires, we ought to expect to find similar 

 tracts east of the Alleghanies. In traversing the great forest 

 adjacent to Lake Superior, where, owing to the resinous nature 

 of the trees, the fires at times rage with unabated fury, consuming 

 even the turf, until quenched by drenching rains, he had seen 

 large areas thus burnt over, but never saw a grassy plain which 

 could be traced to such a cause. In order to fully comprehend the 

 origin of these vast savannas, and to trace that origin to the 

 operation of known laws, it becomes necessary to consider the 

 varying distribution of moisture in connection with the geological 

 distribution of plants. North America may be divided into five 

 zones of vegetation, founded not so much on its botany as on its 

 physical features. 1. The region of mosses and saxifrages. 2. 



